GPS World

Tag: L2C

  • Directions 2023: Advancing GPS to Meet the Future

    Directions 2023: Advancing GPS to Meet the Future

    GPS is the gold standard for precise positioning, navigation, and timing (PNT), impacting the lives of more than six billion users worldwide. The United States economy alone depends on the free, government-provided service across 900 million GPS receivers supporting vehicle navigation systems, general aviation, financial transactions, the electrical grid, precision agriculture, surveying and construction. The GPS enterprise must remain consistent and reliable, while keeping pace with emerging technology without interruption for the end user.

    Space Systems Command (SSC) at Los Angeles Air Force Base in El Segundo, California — the U.S. Space Force’s space development, acquisition, launch and logistics field command — is responsible for maintaining and modernizing the GPS enterprise. The enterprise consists of three segments: the space segment, the control segment and the user segment. Each achieved specific milestones during an exciting and productive 2022.

    Military people navigating on battlefield
    A new MGue for warfighters is moving closer to completion. (Image: EvgeniyShkolenko/iStock /Getty Images Plus/Getty Images)

    Space Segment

    There are currently 37 GPS satellites on-orbit with 31 set healthy. The constellation requires 24 operational satellites for worldwide coverage and a receiver needs to receive transmissions from four of them to determine its position in three dimensions. GPS continues to operate impressively with an average 45-cm accuracy throughout the past year with the most precise day on record at 31.5 cm. The space segment of GPS modernization focuses on GPS III and GPS IIIF satellite development with significant milestones rounded out in 2022.

    For GPS III, after the successful launch of Space Vehicle 5 (SV05) on June 17, 2021, it was set healthy (usable) on May 25, 2022. The significance of SV05 is its full operational capability of the improved civilian L2 (L2C) signal. L2C improves service speed for commercial users via access to two frequencies, improves accuracy when combined with legacy civil GPS signals (L1 C/A), and is less susceptible to ionospheric interference. SV05 is the 24th satellite enabled with the Military Code (M-code), providing worldwide M-code coverage. M-code is designed to give military receivers better defense against jamming, improved accuracy, a more secure and flexible cryptography architecture, and the ability to detect and reject false signals.

    On Jan. 18, 2023, SV06 successfully launched into orbit aboard a SpaceX Falcon 9 Block 5 rocket from Cape Canaveral Space Force Station, Florida. The launch of SV06 marks a key step in the larger goal of modernizing the GPS constellation. Additionally, the 10th and final satellite in the GPS III fleet finalized production and has a target launch date of 2026. GPS III Space Vehicles 7–10 are in storage and available for launch.

    The next generation of GPS satellites continues development. The October 2022 contract award for GPS III Follow-On (GPS IIIF) satellites will onboard additional capabilities. In addition to introducing new civil signals designed to enhance search-and-rescue efficacy and aviation safety, laser retroreflector array for precise ranging, and a fully digital navigation payload, the GPS IIIF satellites will offer a new Regional Military Protection (RMP) capability providing up to 60 times greater anti-jamming measures. A new port on the Lockheed Martin LM2100 Combat Bus supports a substantial increase in flexibility, providing rapid integration of payloads in response to emerging threats in space.

    GPS Enterprise interrelated segments. (Image: Space System Command)
    GPS Enterprise interrelated segments. (Image: Space System Command)

    Control Segment

    The Next Generation Operational Control System (OCX) will replace the current GPS Operational Control System (OCS), supporting the latest U.S. Department of Defense standards and practices for cybersecurity. The updated system includes a modernized and expanded monitor station network, improved anti-jam capabilities, and enhanced operational capability to control modernized military signals.

    In March 2022, OCX completed its fourth and final legacy ground antenna element (LGAE) installation on Kwajalein Island in the Republic of the Marshall Islands. OCX Block 1 and 2 are undergoing Hewlett Packard (HP) Formal Qualification Test (FQT). This event will qualify much of the system’s previously certified mission software functions. The event will also demonstrate system maturity and readiness for system acceptance, operator training, and specific developmental testing milestones with both GPS space and user segments.

    The next-generation control system, OCX 3F, will modify OCX Blocks 1 and 2 to use the enhanced capabilities of GPS IIIF satellites. OCX 3F received Milestone B and Acquisition Program Baseline (APB) approval from the Milestone Decision Authority (MDA) and was authorized to enter the Engineering and Manufacturing Development (EMD) phase in May. In November, the OCX 3F program deployed 3F mission software into OCX’s Near Operations Environment (NOE) for the first time after completion of the program’s first Integration Readiness Review (IRR). The IRR event ensures that the software meets integrity standards and receives approval to be integrated and tested on the NOE prior to software releases to the operational users. OCX 3F anticipates achieving operational acceptance in 2027.

    The GPS III government and industry team recently core mated GPS III SV10 and nicknamed it “Hedy Lamarr” after the actress and inventor. (Image: Lockheed Matin)
    The GPS III government and industry team recently core mated GPS III SV10 and nicknamed it “Hedy Lamarr” after the actress and inventor. (Image: Lockheed Matin)

    User Equipment Segment

    Among the arsenal of GPS user equipment, very few pieces have the technology to use the M-code signal. Maintaining a competitive advantage against the adversary requires use of these signals; the GPS Enterprise is focused on developing Modernized GPS User Equipment (MGUE) capable of accessing these signals. The MGUE program is a joint service program developing modernized M-code-capable military GPS receivers. The program is broken into two increments (Inc 1 and Inc 2). Both are designed to deliver secure PNT performance, allow navigation warfare operations, enhance anti-jam, enhance anti-spoof and anti-tamper, and enable Blue Force Electronic Attack.

    As part of the multiple elements under the MGUE Inc 1 umbrella, L3Harris delivered its final Build 7 ground card to the government on Nov. 16, 2021, and completed regression testing on that kit in February 2022. The final Delta Security Certification and Approval were completed on April 13 and April 29, 2022, respectively. Development of the L3H Ground-Based GPS Receiver Applications Module (GB-GRAM-M) card, which delivers geolocation and precise positioning capabilities for space-constrained applications while providing increased security and anti-jam capabilities, is complete and available for services procurement. MGUE Inc 1 completed qualification testing for the aviation and maritime cards on Sept. 9, 2022, with updated software builds. This build allows the program to progress to 98% of the requirements verified and enables B-2 Bombers and Guided Missile Destroyers (DDG) to continue progress toward operational testing. Completion of this commitment means significant progress toward operational testing for stakeholders and warfighters.

    MGUE Inc 2 held Preliminary Design Reviews for the Miniature Serial Interface (MSI) in summer 2022, bringing the project another step closer to finalizing the EMD phase. Once all closure and action items are completed for the reviews, the government will consider each event complete. Critical Design Review (CDR) is scheduled for this summer and will validate the system design and the ability to meet system performance requirements. MGUE Inc 2 continues to execute the second competitive objective under Phase I for the Joint Modernized Handheld component; the effort is moving closer to completion of the handheld prototype and will ultimately make for a more seamless transition to operations.

    GPS ground antenna at Schriever Space Force Base in Colorado. (Image: U.S. Air Force)
    GPS ground antenna at Schriever Space Force Base in Colorado. (Image: U.S. Air Force)

    Conclusion

    The SSC’s mandate is paramount to maintaining our modern way of life. The space professionals dedicated to developing GPS technology are committed to delivering advanced capabilities to the warfighter, the civil sector, and the world. An interconnected world is ready for us. We’re on our way.

    SSC is the U.S. Space Force field command responsible for acquiring and delivering the capabilities needed by warfighters to protect our nation’s strategic advantage in and from space. It manages an $11B budget for the U.S. Department of Defense and works in partnership with joint forces, industry, government agencies, academic and allied organizations to outpace emerging threats.

    For analogous updates on the other three GNSS constellations, please see:

  • GPS III ‘Magellan’ starts signal transmission

    By Peter Steigenberger, Steffen Thoelert, Oliver Montenbruck and Richard B. Langley

    The first GPS III satellite, “Vespucci,” was launched in December 2018, started signal transmission in January 2020, and was set healthy later that month. The second GPS III satellite, nicknamed “Magellan,” was launched on Aug. 22, 2019, on a Delta IV rocket from Cape Canaveral, Florida.

    Magellan, also identified by its space vehicle number (SVN) 75 (here referred to as GPS-75), started signal transmission with standard pseudorandom noise code (PRN) number 18 (here referred to as G18) on March 13. The L1 C/A, L1 P(Y), and L2 P(Y) signals were activated at 17:16:30 GPS Time (GPST), while the L1C, L2C and L5 signals followed less than two hours after Vespucci’s launch at 18:59:30 GPST. Transmission of navigation messages started at 19:00:00 GPST with GPS-75 (G18) marked as unhealthy.

    PRN G18 was previously used by the 27-year-old Block IIA satellite GPS-34 that had been already removed from the active GPS constellation on Oct. 7, 2019, but continued signal transmission until March 9, 2020. GPS-75 is already being tracked by a large number of tracking stations of the International GNSS Service (IGS). Based on the data collected by these stations, the Center for Orbit Determination in Europe (CODE), headquartered in Bern, Switzerland, has been providing precise orbit and clock products for this satellite since March 14.

    A comparison we performed with the CODE precise orbit products revealed initial broadcast ephemeris errors of up to 100 meters (3D) and an orbit-related signal-in-space range error (SISRE) of about 13 meters. Within four days, a SISRE (orbit component) of 24 centimeters was achieved, which closely matches the performance of the rest of the GPS constellation.

    Figure 1 shows the spectral flux density of GPS-75 in the L1, L2 and L5 frequency bands obtained with the 30-meter high-gain antenna of the German Aerospace Center (DLR) located in Weilheim, Germany. The civil L1 C/A, L1C and L2C signals can be identified as sharp peaks in the center of the respective frequency bands.

    FIGURE 1. Spectral flux density of GPS-75 measured with DLR’s 30-meter high-gain antenna. (Figure: Steigenberger, et al)
    FIGURE 1. Spectral flux density of GPS-75 measured with DLR’s 30-meter high-gain antenna. (Figure: Steigenberger, et al)

    The prominent side lobes in the L1 and L2 bands are associated with the military M-code. The wide main lobe of the L5 signal with two smaller and sharper side lobes is caused by the superposition of two in-phase and quadrature signals with a 10-MHz binary phase-shift keying (BPSK) modulation. We found that all signals are in good shape and have a quality similar to that of the first GPS III satellite.

    On March 16, 2020, we detected a significant change in the carrier-to-noise-density ratio of the L1 and L2 P(Y)-code signals. Figure 2 illustrates these changes for the IGS station located in Patumwan, Thailand (CUSV00THA). The L1 and L2 P-code signals are usually encrypted with the W-code to prevent spoofing (the generation of fake signals by adverse parties). The resulting encrypted signals are denoted by P(Y). Geodetic GNSS receivers are capable of tracking the P(Y) signals with a semi-codeless approach.

    FIGURE 2. Carrier-to-noise-density ratio (C/N<sub>0</sub>) of the second GPS III satellite, GPS-75, tracked by the IGS station CUSV00THA in Patumwan, Thailand, on March 16, 2020. Between 20:22 and 21:18 GPST, unencrypted P-code signals were tracked. (Figure: Steigenberger, et al)
    FIGURE 2. Carrier-to-noise-density ratio (C/N0) of the second GPS III satellite, GPS-75, tracked by the IGS station CUSV00THA in Patumwan, Thailand, on March 16, 2020. Between 20:22 and 21:18 GPST, unencrypted P-code signals were tracked. (Figure: Steigenberger, et al)

    As a result, C/N0 of L1 P(Y) and L2 P(Y) are virtually identical and significantly smaller than the C/N0 of the unencrypted signals due to losses of the semi-codeless tracking technique. This can be seen in the blue-colored plot of Figure 2, where the C/N0 values of L1 P(Y) and L2 P(Y) are identical and smaller by 4.5–16 dB compared to L1 C/A depending on the elevation angle of the satellite.

    However, between 20:22 and 21:18 GPST, an increase of the P-code C/N0 values was observed. The values changed by 4.5 and 12.5 dB for L1 and L2, respectively. This change is an indicator that unencrypted P-code signals were transmitted, rather than encrypted ones. This assumption can be verified by the “Anti-Spoof Flag” given as the 19th bit of the handover word (HOW) of the GPS LNAV navigation message.

    Indeed, decoding of the raw navigation data from the IGS station CHOF00JPN in Chofu, Japan, showed that the Anti-Spoof Flag indicated a deactivation of anti-spoofing between 20:22:00 and 21:17:48 GPST and verified our assumption that unencrypted P-code signals were transmitted during that time period.

    It has to be noted that only Javad receivers within the global multi-GNSS network of the IGS show this increase in C/N0. Other receiver types report continuous C/N0 values for the P-code signals, indicating that a semi-codeless tracking technique was continuously applied irrespective of the Anti-Spoof Flag.

    Figure 3 shows the two GPS III satellites’ Allan deviation, which measures the clock stability achieved in orbit; that is, the average frequency error over different time scales. In addition, the Block IIF satellite GPS-63 is shown, which is in the same orbital plane as GPS-75.

    FIGURE 3. Allan deviation of the Block IIF satellite GPS-63 and the GPS III satellites GPS-74 and GPS-75 computed from 5-minute clock solutions produced by DLR. (Figure: Steigenberger, et al)
    FIGURE 3. Allan deviation of the Block IIF satellite GPS-63 and the GPS III satellites GPS-74 and GPS-75 computed from 5-minute clock solutions produced by DLR. (Figure: Steigenberger, et al)

    For integration times up to 2,000 seconds, the clock stability of GPS-75 is slightly better compared to the first GPS III satellite, GPS-74, but the situation is opposite for integration times larger than 5,000 seconds. The latter finding might be caused by the fact that GPS-75, unhealthy at the time, was tracked by a smaller number of stations compared to the healthy GPS-74.

    As a consequence, the observed Allan deviation may partly be contaminated by orbit determination errors. In any case, both GPS III satellites clearly outperform the Block IIF satellite GPS-63 that suffers from thermal line bias variations visible as an increased Allan deviation starting at an integration time of about 2,000 seconds.

    The activation of the second GPS III satellite transmitting the new civil L1C signal enables the estimation of differential code biases (DCBs) between, for example, the L1 C/A signal (Receiver Independent Exchange [RINEX] format observation code C1C) and different tracking modes of the L1C signal. Septentrio receivers track only the pilot component of the L1C signal (C1L), whereas Javad and Trimble receivers perform a combined data+pilot tracking (C1X).

    DCBs are estimated from pseudorange (code) observations of a global tracking network and are corrected for ionospheric delays obtained from global ionosphere maps. The DCB estimates shown in Table 1 are based on eight days of data from 10 Javad, 21 Septentrio and 3 Trimble receivers.

    TABLE 1. Differential code bias estimates in nanoseconds between L1 C/A and L1C for the GPS III satellites and average receiver DCBs. (Data: Steigenberger, et al)
    TABLE 1. Differential code bias estimates in nanoseconds between L1 C/A and L1C for the GPS III satellites and average receiver DCBs. (Data: Steigenberger, et al)

    As we have applied a zero-sum condition for the estimation of satellite DCBs of just two satellites, the values of GPS-74 and GPS-75 obtained from the same type of L1C observables differ only by the sign. The DCBs estimated from different L1C observables, namely C1L and C1X, differ by 56 picoseconds, corresponding to a range difference of 1.7 centimeters. The receiver DCBs are quite homogeneous for receivers from each manufacturer but differ by up to 6 nanoseconds between various manufacturers.

    On April 1, 2020, GPS-75 was set healthy and joined the constellation of operational GPS satellites. The third GPS III satellite, named “Columbus,” was shipped to the Cape Canaveral launch site in February 2020. Its launch is expected no earlier than June 30, 2020, and at least two GPS III launches per year are planned for the near future.

    Equipment. Measurements reported in this article were collected with JAVAD GNSS TRE_G3TH and TRE_3, Septentrio PolaRx5 and Trimble Alloy multi-GNSS, multi-frequency receivers. The spectral overview was captured with a Rohde & Schwarz EM100 digital compact receiver.

    PETER STEIGENBERGER and OLIVER MONTENBRUCK are scientists at the German Space Operations Center of the German Aerospace Center (DLR). STEFFEN THOELERT is an electrical engineer at DLR’s Institute of Communications and Navigation. RICHARD B. LANGLEY is a professor at the University of New Brunswick and editor of the “Innovation” column for GPS World magazine.

    Further Reading

    “Optimum Semicodeless Carrier-Phase Tracking of L2” by K.T. Woo in Navigation, Vol. 47, No. 2, 2000, pp. 82-99, doi: 10.1002/j.2161-4296.2000.tb00204.x.

    Interface Specification IS-GPS-200K: NAVSTAR GPS Space Segment/User Segment Interfaces by Global Positioning Systems Directorate Systems Engineering & Integration, Los Angeles Air Force Base, El Segundo, California, March 4, 2019. Available online: https://www.gps.gov/technical/icwg/IS-GPS-200K.pdf

    “Apparent Clock Variations of the Block IIF-1 (SVN62) GPS Satellite“ by O. Montenbruck, U. Hugentobler, E. Dach, P. Steigenberger and A. Hauschild in GPS Solutions, Vol. 16, No.3, 2012, pp. 303-313, doi: 10.1007/s10291-011-0232-x.

    “Differential Code Bias Estimation Using Multi-GNSS Observations and Global Ionosphere Maps” by O. Montenbruck, A. Hauschild and P. Steigenberger in Navigation, 2014, Vol. 61, No. 3, 2014, pp. 191-201, doi: 10.1002/navi.64

  • L2C and Next-Generation Smart PNT Receivers

    L2C and Next-Generation Smart PNT Receivers

    Are you using a legacy-model PNT (position, navigation and timing) receiver or a smart PNT receiver, and why does it matter? Don’t have a clue? Read on! Hint — L2C and CNAV (civilian navigation message format) are the major reason it matters. Yes, it’s all because of L2C, the controversial GPS civilian signal that seems to always be in the news and just keeps getting better the more we learn about it.

    It was just 30 months ago that I penned a column titled 2C or not 2C: An Important Signal Question.

    A couple of weeks ago, Alan Cameron, our esteemed editor in chief — penned a follow-on editorial comprised of excerpts from techies, subject-matter experts and editors, including yours truly, exchanging opinions about the flexibility, sustainability and capability of the GPS L2C signal and all that signal enables.

    I won’t bother to go into the details or history of the L2C signal here, as I did that in excruciating detail 30 months ago. However, let’s consider L2C 30 months on and determine if the landscape has changed.

    What is L2C?

    According to the official U.S. government PNT website, “L2C is the second civilian GPS signal, designed specifically to meet commercial needs.” As it turns out, the military needs L2C as much as the civilian world, but that is a story for another time. When combined with L1 C/A (coarse acquisition signal) in a dual-frequency GNSS receiver, L2C enables ionospheric corrections, a technique that boosts accuracy. Civilians with dual-frequency GPS receivers typically enjoy the same or better accuracy as the military.

    For professional and high-precision users with existing dual-frequency receivers, L2C delivers faster signal acquisition, enhanced reliability and greater operating range. L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it more jam and interference resistant, plus it’s easier to receive signals under trees and indoors. The U.S. Commerce Department estimates L2C will generate about $6 billion in economic productivity benefits through the year 2030. Considering there are more than four billion GPS users around the world today, the DOC economic benefits number seems rather low.

    L2C Status

    The first GPS IIR-M (R= Replenishment, M= Modernized with M-code and L2C) satellite featuring L2C launched on Sept. 26, 2005, and is still operational today. Every GPS satellite fielded since then (18 SVs, including SVN 49) has included an L2C transmitter. This equates to 16 operational L2C satellites on orbit and transmitting, with GPS IIF-10 being number 17 when it is fully commissioned. With 17 SVs (GPS satellite vehicles) on orbit, the L2C system is officially near Initial Operating Capability (IOC). With the requisite ground system upgrades, which are in the works, this means that on any given day most users will have at least one or more L2C signals in view. You can be sure manufacturers will be quick to take advantage of the geometry.

    SVN49 in space (artist’s rendering). The signal anomaly from SVN 49 alerted researchers to new possibilities in analysis and monitoring.
    LMCO GPS IIRM Satellite Vehicle On Orbit. (Artist’s rendering courtesy of Lockheed Martin)

    Legal Caveats

    “In April 2014, the U.S. Air Force began broadcasting civil navigation (CNAV) messages on the L2C and L5C signals. Prior to that time, L2C and L5C provided a default message or Message Type Zero, containing no data. Adding additional CNAV message types required upgrades to the GPS control segment. On Dec. 31, 2014, the Air Force began transmitting CNAV uploads on a daily basis. L2C should continue to be considered pre-operational and should be employed at the user’s own risk.”

    Now the lawyers are happy.

    So What?

    What does this mean for the average user? You might be surprised at the answer. Depending on how technical you are and exactly how you use GPS, it could mean that all your “legacy” GPS receivers are about to become obsolete. Or, depending on the company that builds your receivers and the amount of foresight they built in, it could just mean a few firmware upgrades and new applications.

    Regardless, with the full implementation of L2C GPS signals and navigation messages, GPS will never be the same again. This is not to say your legacy receiver will not work just as efficiently as it does today, and in fact you will probably be able to use it quite effectively for years. But it will not be able to take full advantage of all the capabilities L2C enables without an upgrade, if indeed it is upgradeable.

    Legacy versus Smart

    No matter how much or how little you paid for your GPS/GNSS/PNT receiver, it is essentially — except for a few notable exceptions — a legacy receiver. For example Trimble is ahead of the game as they began producing L2C capable receivers as early as 2003 and are just waiting for the additional L2C messages to be defined. Again, those receivers that are not L2C-ready or capable are what I will classify as a legacy receiver, simply because of all the future capabilities that are missing. Your current PNT receiver may have the potential to be a smart receiver — it may have the technical capability to process far more than it does today. But, unfortunately, essentially almost every receiver, again with a few exceptions, on the market today falls into the “legacy ” category.

    Is My Legacy Device Considered Obsolete?

    Now that I have your attention and have probably riled more than a few GPS device manufacturers, please allow me to explain. In the past, your GNSS/PNT device (for brevity’s sake, I will default to PNT for the rest of the column) has basically performed a simple function. It displayed your position, and perhaps maps and other ancillary data (targets or destinations) after it received, decoded, verified and applied timing signals and a very small number of navigation messages.

    It accomplished this feat typically from a cold start in under 120 seconds. Maybe much less. Recently, I was privileged to view a demonstration of a receiver from a major manufacturer that performed a warm restart in less-than-ideal conditions and displayed a useful position in 1/20th of a second. As amazing as that may be, it is still today classified as a legacy receiver. It accomplished its task; it supplied a useful position both in human and machine language that could be utilized by both. In the past, this was the task your receiver accomplished routinely. With the full implementation of L2C, all that changes and changes drastically. I call it a revolution for PNT, but alas I am frequently given to hyperbole. However, give me a moment and see if you don’t agree.

    I was attracted to a Wall Street Journal headline recently by a company that I know well, since they have an abundance of well-known and multi-talented former military leaders. That company, Accenture, puts it this way: “Change is good. Transformation is even better.” That is exactly where I believe we stand today with L2C. It is a game changer.

    For example, just this week in the WSJ, which I read cover to cover six days a week, I saw stories about Audi vehicles driving autonomously from coast to coast, over 3,000 miles without driver intervention. Contrary to many manufacturers, Audi is quick to credit GPS with a large portion of the proprietary Audi (VW) technology and the capability it enables. There was a story about commercial vehicles, over-the-road diesel trucks that may have even more capabilities than the Audi. Again, with GPS as the prime contributor. The same WSJ story mentioned that, “Some of the features being added to trucks are similar to those in cars, but generally the move to autonomy in commercial and industrial vehicles is far ahead of the autonomous systems offered on most passenger vehicles. Already, mining vehicles and military forklifts are operated without drivers.”

    Amazingly, these capabilities depend greatly on GPS, but exist without the full implementation of the revolution that L2C, CNAV and multiple nav messages will bring.

    L2C CNAV Message Structure.
    L2C CNAV Message Structure.

    L2C Ready

    I have over the past year seen advertisements for PNT devices that proclaim they are L2C ready. I beg to differ, but only because my definition of L2C ready probably varies greatly from that of the devices’ marketing department. Beyond its signal structure, L2C has a new messaging capability.

    As stated earlier, the L2C signal is heads and shoulders above most other GNSS signals in strength, code structure and security. L2C delivers faster signal acquisition, enhanced reliability, and greater operating range. L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it interference and jam resistant and easier to receive under trees and indoors. These attributes make it a great signal and when you consider the carrier-phase and RTK (real time kinematic) capabilities, which really are real-time today. It is a very appealing signal indeed.

    For precision and timing users, the carrier phase of the L2C signal, non-coded carrier, is 1,000 times more stable than the fully coded L2C signal. The L2C carrier-phase stability will remain unchanged until the semi-codeless transition date of Dec. 31, 2024, per the FRP or Federal Radio Navigation Plan of 2014. Then officially all bets are off, but who knows? That date could be extended.

    However, the real and future strength of the L2C signal structure is hiding in one or more (accurately 255 more, for a total of 256) messages that can be utilized in a myriad of ways and applications. These are messages, nav-messages if you will, that your new or updated PNT device will be able to utilize for who knows how many functions. Just use your imagination. Here are some ideas I have for using the additional L2C messaging capability.

    • Send 250+ other navigation messages, to be defined.
    • Send continuous atmospheric corrections (such as ionospheric) for each two degrees of longitude around the globe or in one degree increments if you consider land mass applications only.
    • John Deere and Trimble as the leading commercial and civil providers of navigation data could appropriate a small fragment of the messages for their global navigation and timing corrections to their agricultural and precision users/customers around the globe.
    • Companies or governments could send nominal navigation or even text-based navigation-related messages to users anywhere an L2C signal can be received.
    • Companies could shut down and render useless receivers from users that have not paid their bills or were abusing the system.
    • Companies could send small firmware updates or notices of larger updates directly to users. Data could include active hyperlinks.
    • Precision, scientific and premium users might have the capability to receive constant correction updates that make their PNT receiver a centimeter or potentially a millimeter level device.
    • Receivers with communications — four billion plus smartphones and other devices with PNT capabilities and built-in communications — could become sensors capable of being sampled at will. These devices have the potential to be considered remote monitoring stations both for PNT and communications purposes. They could report both communications and PNT jamming or interference. They could also help track intentional jammers.

    If you think about it hard enough, you will see that this modest list of capabilities with the proper security either make spoofing an impossibility or without proper security a malicious nightmare.

    I hope by now you catch my drift and have come up with some ideas of your own concerning how the additional 250+ L2C messages could be utilized. We’re unsure how many messages will actually be available or how the messages will be used. The government will, out of operational necessity, require a small number, so right now your guess is as good as mine.

    Keep in mind that L5C and M-code will have the same capabilities on differing frequencies, and different governing bodies will decide how the signals and 750-plus multiple-messaging capabilities are allocated and utilized. That is all hopefully in the near future. How that process unfolds, technically and operationally, will have a great deal to do with how successful and ubiquitous L2C becomes. The process alone will undoubtedly spawn thousands of articles; however, right now we are primarily discussing the necessity for smart receivers to fully utilize the additional L2C messages. For along with all the potential capabilities comes a processing and communications tail that does not exist today, except in a few instances that we can’t go into in this venue.

    Relative

    This is probably a good time to further qualify what I mean by legacy versus smart receivers. Were the appellation “legacy” not already in our vernacular concerning today’s highly functioning devices, it would not be one I would have chosen. However, it is and we are stuck with it. Consider that there are static high-end (read premium quality) single GNSS receivers that “see” more than 50-60 separate GNSS satellite vehicles and processes more than 150 GNSS signals. This does not take into consideration all the augmented and companion signals some of these devices are capable of processing. Many of these devices are very difficult to jam and literally cannot be spoofed, and still today they are legacy receivers in relationship to L2C capabilities.

    However, I am told such high-end receivers are absolutely L2C ready, which may mean the additional L2C messages are ready to be processed and applied, received or rejected, whenever they are properly and officially defined. This brings us to the future definition or next generation smart L2C receiver.

    Smart L2C PNT Receiver

    For the first time a smart PNT L2C capable receiver will have the ability to:

    • Select between GPS only, GPS + GLONASS, or full GNSS mode with ancillary corrections such as WAAS and EGNOS, and work with, process or reject messages, making a decision about some or all the signals it has in view. While there are receivers that accomplish some of these functions today, they do not typically have the option of accepting or rejecting a GPS navigation message if it is properly formatted and verified. L2C smart receivers will — indeed must — at a minimum possess and correctly utilize that capability.
    • Alert users concerning new navigation message(s) and determine automatically or with user input whether the navigation message should be applied immediately, in the near future, put on hold or totally rejected.
    • Alert users to the effect that applying new or multiple navigation messages will have on the current PNT display and possibly the current mission or operation. For example, if you are a precision user, think millimeters for level of accuracy, utilizing PNT to measure tectonic plate movement — you are very interested in relative displacement over time and you may have no desire to apply a multiple nanosecond correction that could move your current measured position several inches or feet. If you are a geocacher, you do not want the coordinates of your latest buried treasure to dynamically change.
    • Determine if the latest valid navigation message(s) apply to your geographic area or, for mobile receivers, your destination, and what effect incorporating the messages will have on your displayed position or ETA.
    • Display a text-based navigation message if it is addressed to your device.
    • Require password(s) for certain actions — be they sensitive, proprietary, classified or of a “cannot undo” nature. Passwords could also be required in the message format before it could be unlocked and applied.
    • Determine and alert users if multiple navigation or device-control messages conflict with organizational or user-defined parameters.
    • Alert users to malicious messages or spoofing attempts.
    • Alert users to GNSS assets that are no longer available or go offline, such as during the two total GLONASS constellation shutdowns when GLONASS signals were not available for several hours. In the case of Apple iPhones, the GLONASS constellation-wide shutdown meant these devices went from multiple GNSS devices to “GPS plus PNT augmentation (WAAS) and other onboard sensors” devices. This is something many users may not care about, but is definitely worth a user-defined parameter for a warning message.
    • The ability to permanently reject a certain type of message by type, source, timeframe, etc.

    By now, I hope you see the trend. You can probably think of many more possibilities for future GNSS or PNT receivers and the necessity for them to be loaded with computing and communications capabilities, especially where L2C is concerned — indeed, where all the CNAV signals and messages are concerned.

    Bottom Line

    The bottom line is L2C is a potentially revolutionary signal for GPS/PNT; it opens incredible opportunities for entrepreneurs, manufacturers and users at a minimum. We now all have some hard and important questions to consider before we purchase our next-generation PNT device or upgrade our legacy device.

    Until next time, happy navigating, and I hope to see everyone at ION GNSS+ in September in Tampa, Fla. Remember, GPS is brought to you courtesy of the United States Air Force.

  • To L2C or Not to L2C? That Is the Operational Question.

    Half of the GPS constellation now transmits the new civil signal, L2C. In a matter of weeks, that number will crest into the majority of the constellation when IIF-10 is set active and operational to users. By the end of the year or early 2016, look for 18 usable satellites transmitting L2C. That could be considered a nominal initial operating capability (IOC), though it is unlikely to be declared as such by the Air Force. We can anticipate a full operating capability (FOC) within five years. Many high-precision GPS receivers currently embody L2C signal processing capability.

    As Oscar Colombo, research scientist at NASA, noted in a recent CANSPACE contributed note, “This seems like a moment to start seriously thinking about using L2C as much as possible.”

    This month’s newsletter presents an amalgam, a panel discussion in virtual print, on several aspects and viewpoints stimulated by his posting,

    Some readers may want to peruse this U.S. government bulletin for a general description of L2C; others who feel sufficiently informed may skip directly two paragraphs down to “Three issues might be in the way of that being a practical proposition.”

    “L2C is the second civilian GPS signal, designed specifically to meet commercial needs. Its name refers to the radio frequency used by the signal (1227 MHz, or L2) and the fact that it is for civilian use. There are also two military signals at the L2 frequency. When combined with L1 C/A in a dual-frequency receiver, L2C enables ionospheric correction, a technique that boosts accuracy. Civilians with dual-frequency GPS receivers enjoy the same accuracy as the military (or better). For professional users with existing dual-frequency operations, L2C delivers faster signal acquisition, enhanced reliability, and greater operating range. L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it easier to receive under trees and even indoors. The Commerce Department estimates L2C could generate $5.8 billion in economic productivity benefits through the year 2030. The first GPS IIR(M) satellite featuring L2C launched in 2005. Every GPS satellite fielded since then has included an L2C transmitter.”

    Oscar Colombo’s CANSPACE note continues:

    “Three issues might be in the way of that [using L2C as much as possible] being a practical proposition, and I would appreciate comments on some or all of them:

    “(1) The fact that the new L2C navigation code (CNAV) is being transmitted, but flagged as pre-operational by the USAF, indicating that this organization is not yet ready to guarantee its fitness for use.

    “(2) The quarter-wave phase difference with the heritage signal L2.
    This one is important to know when fixing the ambiguities of differential observations (double differences and first order differences between satellites) combining L2C data from IIR-M and IIF satellites with those with only L2 (IIR and IIA). Some high-end commercial receivers correct for this phase difference, some don’t.  The latest RTCM document I’ve seen that touched on this issue came out in 2013 (RTCM Standard 10403.2, Paragraph 3.1.8, Table 3.1-5), and listed the choices , at that time, by nine leading manufacturers on this matter. The list does not include all of present-day manufacturers of high-end receivers, a list that changes over time.

    “(3) There is no proper place for L2C in files in the widely used Rinex 2.11 format.
    In principle, this can be taken care of by using data files in the Rinex 3 format. However, the use of Rinex 3, that has some major departures from 2.11, is not universal yet.

    “Does anyone  know of an up-to-date, reliable and comprehensive list of receiver manufacturers showing those that correct and those that do not correct for the quarter-wave phase shift?”

    GPS World contributing editors Eric Gakstatter (Geospatial Solutions) and Don Jewell (Defense) had a private conversation about the above, which I now make public.

    Don Jewell: “I can address this from a policy and operational perspective but you [Eric] have a better feel for the users perspective.

    “With two more successful IIF launches there will then be 18 L2C SVs broadcasting that signal, and that is considered by the government to be nominal IOC, an initial operating capability. Regardless of where you are on the Earth, shy of 60 deg N and 60 deg S, you should always have at least one or more L2C SVs in view.

    “We are probably looking at 2023 (8 more years) before the L2 carrier phase is in jeopardy of shifting without notice. If indeed that ever happens.

    “So from an operational and providers (HQ AFSPC, 50SW and 2SOPS) perspective, certainly the L2C signal should be useful and reliable. Just not normally guaranteed until FOC or full operational capability is declared, usually with 24 SVs broadcasting L2C. With no premature losses that will be halfway through the GPS III launch schedule ~ 2019-20.

    “Schedules are dynamic and always subject to change of course.”

    [Editor’s note: The L2 carrier isn’t going to go away or shifting to another frequency. What might go away is the P(Y) modulation on the L2 carrier if the DoD considers the P(Y) signals redundant once the M-code is fully embraced. If the P(Y) signal on L2 is no longer transmitted, then civil receivers currently using the P(Y) signal to obtain L2 carrier-phase measurements will no longer be able to do this.]

    Eric Gakstatter replied, “I’ve heard that some manufacturers say they are taking advantage of L2C when there are IIRM or IIFs in view and maybe some of the receivers I’m using are doing so. I’ve not paid attention to it.

    “It could be helpful in areas where users are trying to work in difficult environments such as near and under tree canopy.

    “In the case of RTK, I would think the reference station would have to broadcast L2C data.”

    A CANSPACE reader provided the following useful reference, which although it dates from 2012, still contains much immutable data: “The most recent view on the situation I have with L2C can be found here.”

    This links to the presentation slides from an American Geophysical Union 2012 Fall Meeting paper, “The Effects of L2C Signal Tracking on High-Precision Carrier Phase GPS Positioning: Implication for the Next Generation of GNSS Systems,” by Frederick Blume, Henry Beglund, and Lou Estey of UNAVCO, a non-profit university-governed consortium, facilitates geoscience research and education using geodesy.

    Oscar Colombo and other CANSPACE subscribers have contributed several further notes t the L2C discussion string.  To read them, it’s possible to access the archives here.
    Or you can more simply and elegantly subscribe to CANSPACE; see instructions here.

    Last month, Richard Langley and Oliver Montenbruck jointly communicated the following interesting aspect of the U.S. Federal Radionavigation Plan to CANSPACE readers:

    “in the new version of the FRP is a new phrasing of the earlier statement on guaranteed availability of the P(Y) signal only up to 2020:

    “The [U.S. Government (USG)] commits to maintaining the existing GPS L1 C/A, L1 P(Y), L2C, and L2 P(Y) signal characteristics that enable codeless and semi-codeless GPS access until at least two years after there are 24 operational satellites broadcasting L5. Barring a national security requirement, the USG does not intend to change these signal characteristics before then. Twenty-four satellites broadcasting the L5 signal is estimated to occur in 2024. This will allow for the orderly and systematic transition of users of semi-codeless and codeless receiving equipment to the use of equipment using modernized civil-coded signals. Note that it is expected that 24 operational satellites broadcasting L2C will be available by 2018, enabling transition to that signal at this earlier date. Civilian users of GPS are encouraged to start their planning for transition now.”

    Finally, Richard Langley notes that “I have a student looking into which Precise Point Positioning engines can currently process L2C observables, but his report is not yet available. Also, we are looking into adding an optional L2C processing capability to the University of New Brunswick PPP software, GAPS (GPS Analysis and Positioning Software), but that’s a month or so away.”

    As a postscript, there is a trending discussion at the LinkedIn group, GNSS R&D, Using C\A acquisition products to acquire L2C long code.

  • Activation of Pre-Operational CNAV Message Set for April 28

    U.S. Air Force Space Command will be implementing CNAV messages on the GPS L2C and L5 signals beginning April 28 at 14:30 UTC, to facilitate user familiarization and development of compatible user equipment. No GPS satellite outages or degradations are planned. The L2C and L5 CNAV messages should be transparent to users.

    A 30-day public comment period on the pre-operational CNAV message ended April 4, apparently without enough concern to halt or delay the implementation.

    These pre-operational signals may not comply with all requirements, and so shouldn’t be used for safety-of-life or other critical purposes, Air Force Space Command said.

    Full text of the NANU appears below:

    Subject: New NANU 2014038
    NOTICE ADVISORY TO NAVSTAR USERS (NANU) 2014038 NANU TYPE: GENERAL
    *** GENERAL MESSAGE TO ALL GPS USERS *** The purpose of this notification is to inform users of an upcoming event related to the GPS satellite constellation.  Air Force Space Command will be implementing CNAV messages on the GPS L2C and L5 signals beginning J118/1430z with updates from the control segment approximately twice per week.  The message populated signal content will include Broadcast Message Types (MT) 10, 11, 12, 30, and 33.  There are no planned GPS satellite outages or degradations for this activity.  L2C and L5 CNAV messages should be transparent to GPS receivers that do not process L2C or L5 CNAV messages.  These populated signals are intended to facilitate user familiarization and development of compatible user equipment.
    NOTE:  Until further notice, the L2C and L5 signals are considered pre-operational. A pre-operational signal means the availability and other characteristics of the broadcast signal may not comply with all requirements of the relevant Interface Specifications and should be employed at the users’ own risk. Therefore these signals should not be used for safety-of-life or other critical purposes.  Any military or civil users who encounter user equipment problems following message population of the L2C and L5 signals should contact the applicable POCs identified below as soon as possible.  Aviation users should file reports consistent with FAA-approved procedures.
    *** GENERAL MESSAGE TO ALL GPS USERS ***
    POC: CIVILIAN – NAVCEN AT 703-313-5900, HTTP://WWW.NAVCEN.USCG.GOV
    MILITARY – GPS OPERATIONS CENTER at HTTPS://GPS.AFSPC.AF.MIL/GPSOC, DSN 560-2541,
    COMM 719-567-2541, [email protected], HTTPS://GPS.AFSPC.AF.MIL
    MILITARY ALTERNATE – JOINT SPACE OPERATIONS CENTER, DSN    276-3514,
    COMM 805-606-3514, [email protected]

  • PNT Advisory Board Hears Air Force CNAV Plan

     

    The U.S. National Space-Based Positioning, Navigation, and Timing (PNT) Advisory Board has published the minutes of its December 4–5, 2013, meeting, opening with a quote from Albert Einstein, “We cannot solve our problems with the same thinking we used when we created them,” courtesy of Board Chair Dr. James Schlesinger. Among many other topics addressed, the Board heard a report from Major General Martin Whelan, Director of Requirements, Air Force Space Command, on the road ahead for implementation of the GPS Civil Navigation (CNAV) message on L2C and L5. The subject has stirred some controversy of late, particularly between the U.S. Departments of Transportation (DoT) and Defense (DoD), and DoT is currently seeking public comments on the plan.

    The meeting minutes relay the gist of General Whelan’s CNAV remarks as follows:

    “While sequestration is having various impacts on DoD budgets, thus far GPS quality, service and refresher plans are unaffected. The FY15 budget is under development.

    “CNAV has been under discussion for a considerable time. Currently, L2C and L5 signals are being transmitted, but without a navigation message. AFSPC is working hard to activate these messages as soon as possible. One of the reasons for the delay is that additional time was needed to complete testing prior to activation. Testing began in late summer 2013 and, based on initial test results, a “way ahead” has been plotted. Gen William Shelton, AFSPC commander, wished to assure the Advisory Board of his unwavering commitment to providing full-time broadcast CNAV messaging capability on L2C and L5 as soon as possible.

    “The CNAV capability will add diversity and robustness for dual frequency users. Gen Shelton intends to provide details plans to the NCO and a report to the next EXCOM meeting. Current plans are to begin initial broadcasting in the spring of 2014. CNAV uploads will occur twice weekly. The signal will meet GPS Standard Positioning System (SPS) standards, but may not achieve current accuracy levels until full implementation in late 2014.

    “CNAV live sky testing occurred in June and was conducted in cooperation with civil, industry, and international partners. The two-week test series included independent assessment and verification. The tests identified four errors that required action. The first, which was addressed in real time, related to implementation of the test series. The second required improvement to the tools suite, which should be totally integrated into the ground segment by December 2014. The third and fourth errors required patches to satellite software. All four issues are now regarded as closed.”

    The meeting minutes report this further discussion of CNAV.

    “Dr. Schlesinger raised the topic of sequestration and how, based on his early career in budgeting, no budget item is sacrosanct. GPS has enjoyed protection from Deputy Secretary of Defense Ashton Carter, but he is now stepping down and his replacement not yet known. This could provide an opportunity for “the men with the green eyeshades” to come forward to eliminate things.

    “Gen Whelan said he agreed that with sequestration, everything – including GPS – is on the table. However, AFSPC continues to strive to avoid any degradation in service. He also welcomed the continued support of the Advisory Board.

    “Dr. Schlesinger quoted from a 2006 document: “Our position is to continue to provide the best space-based positioning, navigation and timing service in the world.” The Chinese are now “moving up” on GPS. How is GPS going to stay ahead?

    “Gen Whelan said AFSPC is aware of China’s steps in capacity and signal diversity. This, however, does not alter his confidence that GPS remains the “Gold Standard” of world GNSS systems. AFSPC is committed to maintain GPS leadership. However, because of sequestration and budget cuts, this position could not be the position of some people outside of the Air Force.”

    A subsequent presentation from the Department of Transportation given by Karen Van Dyke, Director for PNT, DOT Research & Innovative Technology Administration (RITA), did not directly mention CNAV, according to the meeting minutes, but did include this update on civil signal monitoring, taken from the meeting minutes.

    “DOT is responsible for performance monitoring of GPS civil signals. She called attention to the International Committee on GNSS’s (ICG’s) transparency principle that “Every GNSS provider should publish documentation that describes the signal and system information, the policies of provision, and the minimum levels of performance offered for its open service.” Currently, this is only done on GPS L1 C/A signals. Performance standards for L2C and L5 have not yet been established. The crucial function of signal/service monitoring is to verify that commitments to GNSS performance are being met. Additionally, monitoring improves the situational awareness for GNSS operators, and provides assurance that any civil service failure is detected and resolved promptly. All these factors support the GPS performance history that has made it the world’s Gold Standard.

    “The DOT “GPS Civil Monitoring Performance Specifications” (CMPS) document defines the measurements required to show if performance standards for monitoring GPS’ signals/service are met. The document’s first version was developed in 2005 and listed 193 requirements, covering performance monitoring, signal monitoring, non-broadcast data requirements, and reporting and archiving requirements. The document was later updated to align with the 2008 GPS SPS Performance Standard. The most current CMPS was completed in April 2009 and is available at GPS.gov. Since 1999, DOT has published quarterly reports providing analysis of SPS performance for the Federal Aviation Administration (FAA).”

    Further Topics

    Other reports delivered to the Advisory Board, and available in the the full meeting minutes, available here,  include the following. In addition, many PDFs of the individual reports  are available through the meetings Agenda page.

    Global Differential GPS System as a Civil Monitoring Utility
    Dr. Yoaz Bar-Sever, Manager, Global Differential GPS System, NASA Net Propulsion Laboratory

    Automated Driving & Safety Considerations (collision avoidance warning, vehicle-to-vehicle communications, and driverless automobiles)
    Russell Shields, PNT Board Member, founder of Ygomi LLC

    GPS Disruptions: Efforts to Assess Risks to Critical Infrastructure
    The Government Accountability Office’s (GAO) Report on Enhancing Interagency Actions
    Eli Albagli, senior analyst, GAO

    2013 National Infrastructure Protection Plan (NIPP)
    Department of Homeland Security Implementation
    Robert Kolasky, Director Strategy and Policy, DHS Office of Infrastructure Protection

    Economic Impacts of GPS on Key Sectors in the U. S. Economy
    Dr. Nam D. Pham, economist/managing partner, NDP Consulting Group

    GNSS Signal Capability – Multi-Constellation Management
    Cross-Correlation of Existing & Evolving C/A System Signals
    Dr. A. J. Van Dierendonck, AJ Systems

    How Far to Take GNSS Interoperability/Interchangeability?
    Ken Hodgkins, Office of Space & Advanced Technology, Department of State.

     

  • Comment Period on Pre-Operational CNAV Message Opens

    A Federal Register Notice has been published allowing for a 30-day comment period on the proposed CNAV message on L2C and L5. The notice seeks comment from the public and industry regarding plans by the U.S. Air Force to broadcast pre-operational L2C and L5 civil  navigation (CNAV) messages from certain GPS satellites beginning in April.

    The Department of Transportation is the agency seeking comments. Its concerns about the plan drew ire in January.

    “These messages will be formatted in accordance with Interface Specifications IS–GPS–200G and IS–GPS–705C, each dated January 31, 2013. However, a pre- operational signal means the availability and other characteristics of the broadcast signal may not comply with all requirements of the relevant Interface Specifications and should be employed at the users’  own  risk,” the notice says.

    According to the notice, the Department of Transportation seeks comments on the benefits, risks, or issues to users from the plan, including comments on the appropriate timeline for broadcasting pre-operational CNAV messages. Comments are requested from industry on:

    • the receiver development benefits and other intended uses of pre-operational signals, and
    • the benefits and potential impacts to users of continuous pre-operational CNAV messages with L2C and  L5 signals set healthy.

    The deadline to submit comments is April 4, 2014.

    Comments should include the docket number [DOT– OST–2014–0028] and be submitted using one of the following methods:

    (1) Federal  eRulemaking Portal: www.regulations.gov.

    (2) Fax: 202–493–2251.

    (3) Mail: Docket Management Facility (M–30),  U.S. Department of Transportation, West Building Ground Floor,  Room W12–140, 1200 New Jersey Avenue SE., Washington, DC 20590–0001.

    (4) Hand delivery: Same as mail address above, between 9 a.m. and  5 p.m., Monday through Friday, except Federal holidays. The telephone number is 202–366–9329.

    The full Federal Register Notice can be downloaded here.

  • Air Force Directs Early Civil Navigation (CNAV) Message-Populated L2C and L5 Signals

    Photo: CNAV

    The U.S. Air Force is directing transmission of continuous CNAV message-populated L2C and L5 signals starting in April 2014. The move is designed to help development of user equipment compatible with the civil signals. Full text of the CNAV memo appears below.

    MEMORANDUM FOR THE NATIONAL COORDINATION OFFICE FOR SPACE BASED POSITIONING, NAVIGATION AND TIMING

    ATTENTION: DR. JAN BRECHT-CLARK

    FROM: AFSPC/CC
    Peterson AFB CO

    SUBJECT: Early Civil Navigation (CNAV) Message-Populated L2C and L5 Signals

    1. I have directed transmission of continuous CNAV message-populated L2C and L5 signals prior to fielding the Next Generation Operational Control Segment. This provision of populated signals will facilitate development of compatible user equipment and a CNAV Operations Concept.
    2. The message-populated broadcast is projected to begin April 2014. Users should expect initial CNAV signal accuracy to be less than the legacy signals. Upon full implementation in December 2014, CNAV signal accuracy should meet or exceed the legacy signals.
    3. Air Force Space Command is committed to ensuring GPS remains the world’s gold standard for PNT services. The user community can anticipate additional information on CNAV message implementation in the months preceding the initial April broadcast.

    WILLIAM L. SHELTON
    General, USAF
    Commander

  • 2C or Not 2C: The First Live Broadcast of GPS CNAV Messages

    By Oliver Montenbruck, Richard B. Langley, and Peter Steigenberger

    Over the past several years, some users of the GPS navigation system have already benefitted from the addition of various new signals in addition to the legacy C/A- and P(Y)-code. With the introduction of the Block IIR-M satellites in 2005, a new civil signal (L2C) was transmitted on the L2 frequency, and a new signal on a new frequency (L5) was introduced as a standard signal with the Block IIF satellites beginning in 2010. These new signals provide direct access to dual-frequency observations and thus enable improved ionospheric corrections for civil, including aeronautical, users. In addition, a new Civil Navigation (CNAV) broadcast message has been defined in the GPS Interface Specifications (IS-GPS-200 and IS-GPS-705).

    This message will be transmitted jointly on the L2C and L5 signals and provides a variety of useful new parameters. Compared to the legacy L1 C/A-code navigation message, the CNAV message also offers an increased flexibility concerning the type, sequence, and repeat rate of specific messages.

    CNAV messages have already been broadcast over the past two years by the Michibiki (QZS-1) satellite of the Japanese Quasi-Zenith Satellite System (QZSS), which shares many aspects of the GPS signal design. In contrast to this, Block IIR-M and IIF GPS satellites have only transmitted dummy messages so far and a fully operational CNAV transmission is only foreseen once the ongoing modernization of the GPS control segment has been completed.

    Triggered by various interest groups, the Global Positioning Systems Directorate has just conducted a first test campaign with live CNAV transmissions on L2C and L5 over the two-week period from June 15 to 29 (see Global Positioning System Modernized Civil Navigation (CNAV) Live-Sky Broadcast Test Plan.) It served as a first opportunity for end users and receiver manufacturers to test the decoding and use of the new messages under a wide range of different configurations.

    CNAV messages have a common length of 300 data bits and are identified by a message type number that signifies their contents. The messages presently defined for GPS are summarized in Table 1. For QZSS, complementary messages have been established, which enable, among other features, a rebroadcast of GPS-specific data to QZSS users.

    Table 1. Summary of CNAV message types transmitted by space vehicles (SVs). Messages marked by an asterisk were transmitted during the recent CNAV test campaign.

    Message

    Type

    CNAV Message Title

    Function/Purpose

    0*

    		 Default Default message (transmitted when no message data is available)

    10*

    		 Ephemeris 1 SV position parameters for the transmitting SV

    11*

    		 Ephemeris 2 SV position parameters for the transmitting SV

    12*

    		 Reduced Almanac Reduced almanac data packets for seven SVs

    13

    		 Clock Differential Correction SV clock differential correction parameters

    14

    		 Ephemeris Differential Correction SV ephemeris differential correction parameters

    15*

    		 Text Text (29 eight-bit ASCII characters)

    30*

    		 Clock, Iono & Group Delay SV clock correction parameters, ionospheric and group delay correction parameters (inter-signal correction parameters)

    31

    		 Clock & Reduced Almanac SV clock correction parameters, reduced almanac data packets for four SVs

    32*

    		 Clock & EOP SV clock correction parameters, Earth orientation parameters; Earth-centered, Earth-fixed to Earth-centered inertial coordinate transformation

    33*

    		 Clock & UTC SV clock correction parameters, Coordinated Universal Time parameters

    34

    		 Clock & Differential Correction SV clock correction parameters, SV clock and ephemeris differential correction parameters

    35*

    		 Clock & GGTO SV clock correction parameters, GPS to GNSS time-offset parameters

    36

    		 Clock & Text SV clock correction parameters, text (18 eight-bit ASCII characters)

    37

    		 Clock & Midi Almanac SV clock correction parameters, midi (mid-accuracy) almanac parameters

    Other than the legacy L1 navigation message, which adheres to a fixed order of subframes, the sequence of CNAV messages can be varied widely to provide users with an optimized set of low latency information and parameters that change infrequently. As a baseline, the two ephemeris message types 10 and 11 are combined with any of the clock-and-auxiliary data messages (types 30 through 37) to provide users with the orbit and clock data of the received satellites. With a transmission duration of 12 seconds per CNAV message on L2C, a minimum of 36 seconds is required to transfer this information to the user if no other messages are transmitted. On L5, which operates at twice the data rate, a new frame is transmitted once every 6 seconds yielding a minimum of 18 seconds for the broadcast of ephemeris and clock data.

    The recent test campaign started at 18:00 GPS Time on Saturday, June 15, 2013, with the transmission of message types 10, 11, 15, and 30 on a first space vehicle (PRN24) and included PRN12 from 18:42 onwards. Other space vehicles were sequentially phased in until all active IIR-M and IIF satellites (except for the recently launched IIF-4 satellite) transmitted CNAV on the supported signals. When the test ended exactly two weeks later (June 29, 18:00 GPST), all participating satellites were transmitting a complex master frame of 15 x 4 = 60 individual messages, which was repeated once every 12 minutes (on L2C). Each minor frame comprised the two ephemeris messages and at least one clock-data message (see Table 2).

    Table 2. Sequence of message types in a CNAV master frame.

    Message Types

    10

    11

    15

    30

    10

    11

    32

    33

    10

    11

    12

    35

    10

    11

    12

    30

    10

    11

    12

    33

    10

    11

    12

    35

    10

    11

    12

    30

    10

    11

    32

    33

    10

    11

    15

    35

    10

    11

    32

    30

    10

    11

    12

    33

    10

    11

    12

    35

    10

    11

    12

    30

    10

    11

    12

    33

    10

    11

    12

    35

    Other messages included a reduced almanac (message type 12) and a text message (message type 15) with dummy content (“THIS IS A GPS TEST MESSAGE.”)

    The CNAV data were recorded by selected multi-GNSS monitoring stations of the German Aerospace Establishment (Deutsches Zentrum für Luft- und Raumfahrt or DLR) and the University of New Brunswick (UNB), which were specifically configured to record raw GPS navigation frames in addition to the normal observation data. The stations are located at Singapore (SIN0); Sydney, Australia (UNX2); Maui, U.S.A. (MAO0); and Hartebeesthoek, South Africa (HRAG); as well as Fredericton, Canada (UNB) and are equipped with either Javad Delta-G2/G3TH or NovAtel OEM6 receivers. Following initial validation, the raw and decoded data from the CNAV test will be made available to interested users through the Multi-GNSS Experiment (MGEX) of the International GNSS Service (see http:/igs.org/mgex) to facilitate the development of user software and suitable data formats (such as an extended RINEX navigation message format).

    The CNAV orbit and clock data were updated once every two hours and offer a slightly higher bit resolution than their legacy counterparts. However, the accuracy of the ephemeris data has not yet been evaluated nor compared to that of the L1 C/A-code navigation data.

    As indicated above, the CNAV data can also provide a particularly compact form of almanac data known as the reduced almanac. It does not offer clock information (that is not normally required for a signal search) and assumes a circular orbit, which reduces the overall accuracy. Still, it can be transmitted (and repeated) in a much shorter time interval than the legacy almanac, which requires a total of 12.5 minutes. Each reduced almanac message (message type 12) provides orbit information for a total of seven satellites and it takes a set of five such messages to convey information for a complete constellation. For the master frame layout described above, the full constellation reduced almanac is repeated twice within 12 minutes on L2C (and half this time on L5).

    Novel types of CNAV data not covered by the legacy navigation message include the differential code biases (also known as inter-system corrections or ISCs), which are required for pseudorange-based positioning with signals other than the legacy P(Y)-code (in addition to the established Timing Group Delay parameter or TGD). An overview of TGD and ISC values broadcast by the various satellites participating in the CNAV test is given in Table 3.

    Table 3. Differential code biases (in nanoseconds) of GPS Block IIR-M and IIF satellites broadcast during the test campaign as part of the message type 30 CNAV messages.

    SV Type

    SVN

    PRN

    TGO

    ISC L1CA

    ISC L2C

    ISC L5I5

    ISC L5Q5

    IIR-M

    48

    07

    -10.71

    -0.84

    6.52

    IIR-M

    50

    05

    -10.24

    -0.32

    5.41

    IIR-M

    52

    31

    -13.04

    -0.55

    7.36

    IIR-M

    53

    17

    -10.24

    -0.84

    6.17

    IIR-M

    55

    15

    -10.24

    -0.47

    5.62

    IIR-M

    57

    29

    -9.31

    -0.76

    5.06

    IIR-M

    58

    12

    -12.11

    -0.76

    6.64

    IIF

    62

    25

    5.59

    -2.07

    -5.24

    -0.38

    -0.87

    IIF

    63

    01

    8.38

    -2.30

    -7.57

    0.38

    2.15

    IIF

    65

    24

    2.79

    -0.26

    -2.27

    2.27

    3.70

    Another important achievement is the provision of Earth orientation parameters (EOP) in message 32, which provides GPS users with access to the celestial reference frame.  EOPs were transmitted during the second test week and updated on a daily basis (see Table 4). Knowledge of these parameters is of particular interest for GPS-based orbit determination and navigation of spacecraft (in low Earth orbit), which is preferably referred to an inertial rather than an Earth-fixed coordinate system.

    Table 4. Daily Earth orientation parameters from the CNAV test campaign (pole coordinates and dUT1 (UT1-UTC) time differences and derivatives).

    Epoch (GPST)

    x_p

    (arcseconds)

    x_p_dot

    (arcseconds per day)

    y_p

    (arcseconds)

    y_p_dot

    (arcseconds per day)

    dUT1

    (seconds)

    dUT1_dot

    (seconds per day)

    June 22, 0:00

    0.13517

    0.00104

    0.39657

    -0.00054

    0.06341

    -0.00046

    June 23, 0:00

    0.13621

    0.00102

    0.39604

    -0.00056

    0.06295

    -0.00049

    June 24, 0:00

    0.13740

    0.00101

    0.39535

    -0.00058

    0.06231

    -0.00053

    June 25, 0:00

    0.13815

    0.00099

    0.39487

    -0.00060

    0.06164

    -0.00063

    June 26, 0:00

    0.13846

    0.00096

    0.39443

    -0.00062

    0.06078

    -0.00067

    June 27, 0:00

    0.13885

    0.00094

    0.39381

    -0.00064

    0.06004

    -0.00067

    June 28, 0:00

    0.13947

    0.00093

    0.39310

    -0.00066

    0.05909

    -0.00063

    June 29, 0:00

    0.13987

    0.00090

    0.39246

    -0.00068

    0.05842

    -0.00053

    Overall, CNAV offers exciting prospects for improved GPS utilization and users may look forward to the next test campaigns, which will tentatively be conducted once every six months.

    As a side note, it should be mentioned that individual satellites could be observed to transmit various types of non-standard CNAV messages as well as CNAV messages with improper data (such as an invalid week count) after the end of the main test campaign. Various receivers in the MGEX network, which were processing the received CNAV messages internally and which put full confidence in their proper contents, were mislead by such information. During the actual test campaign, all data appeared fully valid and no problems were reported by the stations.

    OLIVER MONTENBRUCK is the head of the GNSS Technology and Navigation Group at DLR’s German Space Operations Center in Oberpfaffenhofen, Germany.

    RICHARD B. LANGLEY is a professor in the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, Fredericton, New Brunswick, Canada.

    PETER STEIGENBERGER is  a staff member in the Institut für Astronomische und Physikalische Geodäsie of the Technische Universität München (TUM) in Munich, Germany.

     

  • GPS CNAV Civil Capabilities Now Being Tested

    CNAV-logoU.S. Air Force Space Command is now testing CNAV capabilities on the GPS L2 and L5 signals. The initial CNAV Live-Sky Broadcast began June 15 and will take place until June 29. Civil users and manufacturers are invited to participate.
    The GPS Directorate (SMC/GP) is implementing the CNAV (Civil Navigation) Test Program, conducting Live-Sky CNAV testing before deployment of the Next Generation Operational Control System (OCX). CNAV is a capability to be provided by Civil Navigation message types for the purpose of improved GPS navigation through the use of signals L2C and L5.

    According to the GPS Directorate, the CNAV Live-Sky testing program will span several years and will evolve to support GPS enterprise and modernized civil navigation performance objectives. Objectives include:

    1. Verify and validate the CNAV requirements specified IS-GPS-200F and IS-GPS-705B.
    2. Facilitate the development of robust IS-compliant L2C and L5 civil receivers.

    Below is the latest Notice Advisory about the upcoming test issued to NAVSTAR users. More information can be found in the NAVCEN test plan PDF.

    NOTICE ADVISORY TO NAVSTAR USERS (NANU) 2013034 NANU TYPE: GENERAL

    *** GENERAL MESSAGE TO ALL GPS USERS ***

    The purpose of this notification is to inform users of an upcoming event related to the GPS satellite constellation.  Air Force Space Command will be
    testing CNAV capabilities on the GPS L2C and L5 signals on Jdays 2013/166-2013/182.  There are no planned GPS satellite outages or degradations for this activity. L2C and L5 CNAV message testing should be transparent to GPS receivers that do not process L2C or L5 CNAV.  These test events will provide an opportunity for civil users and manufacturers to participate in L2C and L5 evaluation. Details on the test plan are located at the NAVCEN web page. Any military or civil users who encounter user equipment problems during or after testing should contact the applicable POCs identified below as soon as possible.  Aviation users should file reports consistent with FAA-approved procedures. Users are cautioned that L2C and L5 signals are not operational.

    *** GENERAL MESSAGE TO ALL GPS USERS ***

    POC: CIVILIAN – NAVCEN AT 703-313-5900, HTTP://WWW.NAVCEN.USCG.GOV
    MILITARY – GPS OPERATIONS CENTER at
    HTTPS://GPS.AFSPC.AF.MIL/GPSOC, DSN 560-2541,
    COMM 719-567-2541, [email protected],
    HTTPS://GPS.AFSPC.AF.MIL
    MILITARY ALTERNATE – JOINT SPACE OPERATIONS CENTER, DSN    276-3514,
    COMM 805-606-3514, [email protected]

     

  • U.S. Air Force to Test CNAV on GPS L2C and L5 Signals

    News courtesy of CANSPACE Listserv.

    U.S. Air Force Space Command has issued a notice that CNAV capabilities on the GPS L2C and L5 signals will be tested in June. No GPS satellite outages are planned. Below is the official notice.

    Notice of Test

    A Notice by the Air Force Department on 03/20/2013

    Action: GPS Test Notice.

    Summary: The purpose of this notification is to inform users of an upcoming event related to the GPS satellite constellation. U.S. Air Force Space Command will be testing CNAV capabilities on the GPS L2C and L5 signals on 15-29 June 2013. There are no planned GPS satellite outages for this activity. The broadcast navigation messages will be in compliance with IS-GPS-200 and IS-GPS-705. L2C/L5 CNAV testing will be transparent to GPS receivers that do not process L2C or L5 CNAV. U.S. Air Force Space Command expects to conduct one to two CNAV tests per year over the next few years. These test events will provide an opportunity for civil users and manufacturers to participate in L2C/L5 evaluation and will result in enhanced provider and user readiness for L2C/L5 operations once the Next Generation GPS Operational Control System comes online in 2016.

    The draft test plan is available. The draft test plan communicates details of the broadcast, data collection, and results reporting plans.

    U.S. Air Force Space Command and the National Space-Based Positioning, Navigation, and Timing Systems Engineering Forum (NPEF) encourage L2C and L5 users and receiver manufacturers to review the test plan, provide comments, and participate in the evaluation process.

    Comments to the test plan must be submitted on a Comment Resolution Matrix by 29 April 2013 and sent to [email protected].

    The final test plan will be posted once all comments have been adjudicated.

    All user and manufacturer comments and the resulting adjudications will also be posted consistent with the GPS public ICWG process.

    Any military or civil users who encounter user equipment problems during or after testing should contact the GPS Operations Center (GPSOC) (military), NAVCEN (civil, non-aviation) as soon as possible. Aviation users should file reports consistent with FAA-approved procedures.

    FOR FURTHER INFORMATION CONTACT:
    Send all questions or concerns regarding the CNAV Test Plan to [email protected].

    Henry Williams Jr.
    Acting Air Force Federal Register Liaison Officer

  • 2C or not 2C: An Important Signal Question

    Don Jewell
    Don Jewell

    With apologies to the Bard of Avon, as I paraphrase his prose, 2C or not 2C — that is the question for millions of GPS users and has been for several years. Indeed, over the past seven years hardly a week has gone by without an email or inquiry about the status of the GPS L2C signal.

    The name “L2C” refers to the radio frequency used by the signal (1227 MHz, or L2) and the fact that it is for civilian use, but of course it will also be used by the military. There are also two military signals on the L2 frequency. In the past couple of months inquiries concerning L2C have become an almost everyday occurrence. Plus I have it on good authority that it was a topic of conversation at recent a PNT EXCOM or Executive Committee Meeting.

    However, when I started making inquiries I was met with an interesting dichotomy. One group of users actually laughed and accused me of having a Don Quixote complex and tilting at windmills, since this debate has been continuing for over seven years, while other more sanguine user groups are seriously interested in the future of L2C. Here is what I discovered.

    What is L2C?

    According to the official U.S. government website concerning GPS, “L2C is the second civilian GPS signal, designed specifically to meet commercial needs. When combined with L1 C/A [ed. coarse acquisition] in a dual-frequency receiver, L2C enables ionospheric correction, a technique that boosts accuracy. Civilians with dual-frequency GPS receivers enjoy the same or better accuracy as the military. For professional users with existing dual-frequency operations, L2C delivers faster signal acquisition, enhanced reliability, and greater operating range. L2C broadcasts at a higher effective power than the legacy L1 C/A signal, making it easier to receive under trees and even indoors. The Commerce Department estimates L2C could generate $5.8 billion in economic productivity benefits through the year 2030. The first GPS IIR(M) satellite featuring L2C launched in 2005. Every GPS satellite fielded since then has included an L2C transmitter.”

    Problems

    So while this would appear to be a “no brainer” as it were, especially the $5.8 billion in economic benefits, it appears the Global Positioning System is actually at a crossroads and the decision makers are apparently on the horns of a dilemma. For some groups of users, the laudable global PNT (position, navigation and timing) gold standard is looking a bit tarnished of late but has an opportunity to shine once again. The question and future perception of GPS by many global users simply revolves around implementing full navigation messages on signals that are already in place, namely L2 and L5 with CNAV and M-code with MNAV. So, what’s the issue, you say? The signals are there, and in the case of L2C and M-code, have been for seven years — simply upload a full navigation message and be done with all the ambiguity that has become unwelcome baggage for these modern day signals. Au contraire, mon ami, if it were only that simple.

    Now grant you this, should be a simple decision, but remember this is the U.S. government we are talking about — apropos failed budgets, endless continuing resolutions, health care fiascos, and the fiscal cliff — important but in the end actually simple decisions all. Unfortunately, history reflects when the U.S. government is involved, nothing is ever easy and timely decisions are hard to come by. So as not to confuse anyone, especially the players, let’s take this process one step at a time and see what all the fuss is about, shall we?

    Problem Statement: The L2C and M-code signal capability has been on orbit for more than seven years, since 2005. In 2009, the 2SOPS began uploading and broadcasting a Message Type Zero on L2C, which, while not a navigation message, does put data on the channel. What users are now asking for is the full navigation message on L2C or CNAV, which is a modern-day navigation message and will have significant advantages for all users. Plus, the signal would be available to potentially more than 3 billion users versus the small subset today that utilizes the L2C carrier phase for RTK (real-time kinematic) operations. Indeed, when compared to the L1 C/A (coarse acquisition) signal, L2C has 2.7 dB greater data recovery (with a navigation message) and 0.7 dB greater carrier tracking.

    The bottom line is that, with the addition of a valid and updated CNAV message on L2C, all users, the military included, would immediately have ten more potential navigation signals available, and that number would grow with each new GPS launch. Obviously, the same is true for military users with M-code and for all users with an L5 capability. However, the upgrade process needs to start somewhere, and L2C is the obvious choice since there are essentially no M-code receivers available today and there are approximately between 450,000-500,000 L2C capable receivers already in the hands of users.

    Assets

    Hardware – Simply put, there are currently 10 satellites (SVs) capable of broadcasting a full navigation message or CNAV message on the L2 signal and MNAV message on the M-code signal, also on L2 and three L5 CNAV capable SVs:

    7 – IIRM SVs (does not include SVN-49 which, while capable, is currently in residual and/or test status).

    3 – IIF SVs (same capabilities as the IIRMs and also capable of broadcasting an L5 signal with CNAV).

    While the lack of L2C monitor stations has historically been used as a reason for not broadcasting the new CNAV navigation messages, today there are literally more than one hundred L2C- and L5-capable monitoring stations, which includes the NASA JPL (Jet Propulsion Laboratory) monitoring stations. Logs from several independent monitoring stations show that at any given time there are from one to three L2C SVs visible for the majority of global users, including our warfighters, who in the mountains of Afghanistan need all the signal availability we can provide.

    Plus, of the estimated 450,000-500,000 fully capable L2C receivers, several thousand of those are in the hands of our warfighters. Trimble started building in the L2C and L5 CNAV capability as far back as 2003, a full two years before the first L2C launch, for users that wanted the option. However, that option has never been fully tested with live on-air CNAV coded transmissions and they need to be. More on testing later.

    From a global user perspective, Japan has essentially leapfrogged the U.S., since the L2C and L5 signals with full CNAV messages are already being broadcast on the Japanese QZSS navigation satellite QZSS-1.

    Software was initially thought to be an issue, but according to my sources, the current MNAV or M-code full navigation message is remarkably similar, as to be almost identical to the CNAV messages for L2C and L5, and MNAV has already been shown to work well on the ten GPS SVs capable of broadcasting the M-code signal. Granted, it is not fully integrated into the current OCS or operational control system known as AEP or Architecture Evolution Plan, but then not many future capabilities are, so this is really not an issue since manual processes abound in the current OCS. The L2C, L5 and M-code full navigation messages are supposedly still part of the plan for OCX or the future operational control system for GPS, which should be up and working sometime in early 2017. This would be twelve years after the L2C and M-code signal capability was first placed on orbit.

    Timelines

    Given those timelines, some would say, hey that’s only four years from now, maybe five if you count testing, and the full navigation message might actually be turned on. But, alas, rumors abound that there is a government agency apparently objecting to the immediate activation of L2C and L5. This agency, and it is not hard to ferret out exactly which one, wants extensive testing to ensure there will be no complications with the WAAS or Wide Area Augmentation System broadcasts, and as an alternative plan it proposes waiting until all the L5 satellites are on orbit before activating navigation messages on L2C or L5. Let’s do the math – that’s 24 L5 SVs, before the L2C and L5 PNT signals are activated with full navigation messages. At one launch per year, the current GPS launch rate, and it matters not whether it is a IIF or a III, and with three L5 SVs already on orbit, that effectively means we will see all 24 L5 SVs on orbit sometime in 2034, if indeed the II-F SVs are truly the first 30-year GPS SVs. If not, then it could be as late as 2037.  However, given the average life span of GPS SVs today, it is more than likely that by 2037 every single L2C, L5 and M-code capable satellite on orbit today will have been disposed of without ever having broadcast any of the new signals for which they were designed. Now, how’s that for a lachrymose plan?

    Fortunately, this alternate long-term “plan’ has not a prayer of passing, dare I utter the words, the dreaded Washington Post test. Never having purposely written anything for the Washington Post, even though several of my articles have been quoted and misquoted there, it is still a force to be reckoned with in our nation’s capital, simply because the Washington Post continually asks the question, “Is this a wise use of taxpayer’s money?” The answer for the alternative GPS L2C and L5 plan is obvious.

    Personnel and monetary issues are always mentioned in the context of new GPS capabilities; however, I have no doubts that the absolutely superb blue suit operators, tacticians and support contractors at 2SOPS are more than capable of performing the simple actions of uploading the new navigation messages on their own.

    Sure, times are tough but the blue suit operators and their on-site support staff, at the 2SOPS, have proven themselves time and again to simply be the best in the business. They have brought GPS SVs back from the dead, they (with Braxton Technologies Software) maintain residual satellites that everyone thought for years was an impossible task, and they have saved SVs for important missions that everyone else thought should have been abandoned. We don’t give the fine men and women at Schriever AFB, the 50th Space Wing and the 2SOPS (2nd Space Operations Squadron) enough credit, and I am not about to sell them short. I am convinced the signals in question can be uploaded with full navigation messages, tested and made operational, even if only on a test basis, within six months without expensive outside help.

    Full-Scale Testing

    This brings us back to the full testing proposal, which actually makes a lot of sense. We (all users) need immediate full scale and extensive operational evaluations that confirm “we have it right” with the CNAV and MNAV data messages. We should encourage manufacturers to participate in this testing process to ensure their GPS devices meet specifications and are not flawed because of a misinterpretation of an interface specification. The GPS equipment manufacturers would be the first to embrace this approach. The manufacturers and users need open-air full-scale testing with valid CNAV messages being broadcast by numerous satellites to ensure they have coded the L2C, L5 and M-code receivers properly.

    Immediate testing is the key here, because otherwise under one proposed plan we will only find out if there is a problem with full end-to-end testing of the L2C CNAV signal and user equipment 13 years after manufacturers began fielding the supposed and promised capability. Under the wait for L5 FOC (full operational capability) plan, we won’t find out if the implementation of specifications are correct for another 20-plus years. Personally, I find this to be unacceptable. We need to begin the immediate six-month test plan now. 

    The aviation community has the mantra, “Test it as you fly it.”  We should not do any less.  A full six-month evaluation is absolutely appropriate and should be undertaken now. We cannot afford to repeat the 5.5C debacle. [ed. click to read more about 5.5C].

    Users, of course, are the prime concern. If you or your organization, including the military, can benefit from 23 more navigation signals today (and three additional signals each year from now until 2034), one of them being the strongest GPS signal to ever be broadcast (L5 with CNAV), and you want/need those signals active now, then please let us know. You may email me at [email protected].

    Stand up and Be Heard

    Although some of my comments have been a bit tongue-in-cheek, this is indeed your chance to be heard and to make a difference. Let there be no mistake: there is no other PNT system in existence today, planned or unplanned, on the drawing boards or on PowerPoint slides that approaches the capability, availability, reliability, longevity, accuracy and integrity of the Global Positioning System. The folks at Air Force Space Command have been and continue to be admirable stewards. We just need to let them go about their business without undue restrictions.

    Dare to Compare

    If you want to know exactly how the GPS compares to other PNT systems, consider that the Russian GLONASS (Globalnaya Navigatsionnaya Sputnikovaya Sistema), which came into existence in 1979 only one year after the first GPS launch, has only been Fully Mission Capable (FOC) as a global PNT system for about two years, and that dubious status is literally day-to-day as it teeters on that magic number of 24 available SVs broadcasting multiple PNT signals simultaneously. Contrast that to the GPS, which currently has 32 active SVs with several additional SVs in residual status. Three GPS SVs are nearing, have attained or will soon attain 20 years on orbit. One of our SVs has been on orbit for more than 22 years. GLONASS has serious problems merely attaining an average SV life of less than five years on orbit. Even the vaunted Chinese are experiencing serious technical, longevity and integrity issues with their multiple PNT constellations. It is just not as easy as it sounds. So yes, GPS is the global PNT gold standard, and its stewards must always strive to improve and lead the way. The GPS must continue to grow and innovate or risk being left behind. This is your chance to contribute to that leadership role. Let your opinion be heard.

    Until next time Happy New Year, happy navigating, and keep those card, letters and emails coming.