CNAV – Page 2 – GPS World

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.

March 12, 2014

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.

March 5, 2014

DoT Disses DoD’s GPS Chops

The departing Deputy Secretary of Transportation, John Porcari, wrote a letter in the closing days of 2013 opposing the U.S. Air Force’s announced plans to begin broadcasting Civil Navigation (CNAV) message-populated L2C and L5 signals as early as April 2014. Military personnel are incensed over what they see as Porcari’s impugning, when not ignoring, the Air Force 35-year track record of broadcasting the gold standard of global navigation satellite signals — something in which Transportation has zero experience.

Porcari alludes in his December 27 letter to “non-standard engineering tools” and “non-standard operations” that he believes would come into play for early CNAV broadcast. “These have the potential to inject human error, which may result in unacceptable GPS constellation operation.”

What Porcari means by “non-standard” he does not specify, although he confesses to unease as “the ability to monitor these signals, [without which] the system will not know if the L2C and LS signals are within specification. Given these risks, DOT is concerned that the CNAV messages could provide hazardously misleading information, impacting GPS safety-of-life, protection of property, and economic security applications.” The full text of the Porcari letter is available here.

In addition to questioning Air Force 2 SOPS ability to broadcast an accurate, compliant signal containing CNAV, the letter appears to ignore — or be ignorant of — the 17 official U.S. government/military monitoring sites for GPS distributed around the world, not to mention thousands of other monitoring sites run by government agencies such as the Jet Propulsion Laboratory, the National Aeronautics and Space Administration, and the National Geospatial-Intelligence Agency, and by many universities such as Stanford, Ohio State, Cal Tech, Massachusetts Institute of Technology, and many other international institutions around the world. Many of these sites collaborate under the rubric of the International GNSS Service.

Finally, two private corporations monitor and correct all GPS signals both from space and on the ground: John Deere and Trimble Navigation. Both companies run commercial, automated GPS signal monitoring systems that that report any glitch, change, power fluctuation, or anomaly in the navigation message for all GPS signals with an average two-second notification time.

“This letter is so much BS,” fumed one source who wished to remain anonymous, “coming from an agency that is in arrears in its GPS payments to the tune of more than $70 million and has no clue how to represent the global GPS user. GPS is a ubiquitous system, not just a tool for the DOT and the Federal Aviation Administration. GPS needs to implement these signals for all users and as a modernization program that was promised to be in place years ago.”

Porcari is leaving for the private sector.

January 9, 2014

The System: Two More Threes for Space

Artist's concept of a GPS III satellite in orbit, courtesy of Lockheed Martin. — Artist’s concept of a GPS III satellite in orbit, courtesy of Lockheed Martin.

Air Force Orders GPS III Satellites 05 and 06 from Lockheed Martin

A December 12 contract modification provided Air Force funding to Lockheed Martin to complete the fifth and sixth GPS III space vehicles (SV 05-06). Lockheeed originally received funding to procure long-lead parts for satellites five through eight (SV 05-08) in February 2013.

The $200,700,415 cost-plus-incentive-fee modification (P00276) on an existing contract (FA8807-08-C-0010) for GPS III space vehicles 05 and 06 means that work will be performed at Littleton. Colorado and Clifton, New Jersey, and is expected to be completed by Dec. 14, 2017 for space vehicle 05 and June 14, 2018 for space vehicle 06. The Air Force Space and Missile Systems Center Contracting Directorate, Los Angeles Air Force Base, California, is the contracting activity.

Galileo Achieves First Airborne Tracking

The European Space Agency’s Galileo satellites have achieved their first aerial fix of longitude, latitude, and altitude, enabling the inflight tracking of a test aircraft.

ESA’s four Galileo satellites in orbit have supported months of positioning tests on the ground across Europe since the first fix in March. Now the first aerial tracking using Galileo has taken place, determining the position of an aircraft using only its own independent navigation system.

The milestone took place on a Fairchild Metro-II above Gilze-Rijen Air Force Base in the Netherlands on November 12. It was part of an aerial campaign overseen jointly by ESA and the National Aerospace Laboratory of the Netherlands, NLR, with the support of Eurocontrol, the European Organisation for the Safety of Air Navigation, and LVNL, the Dutch Air Navigation Service Provider.

A pair of Galileo test receivers was used aboard the aircraft, the same kind employed for Galileo testing in the field and in labs across Europe. They were connected to an aeronautical-certified triple-frequency Galileo-ready antenna mounted on top of the aircraft.

Tests were scheduled during periods when all four Galileo satellites were visible in the sky. The receivers fixed the plane’s position, as well as determining key variables such as the position, velocity, and timing accuracy; time to first fix; signal-to-noise ratio; range error; and range–rate error.

Testing covered both Galileo’s publicly available Open Service and the more precise, encrypted Public Regulated Service, whose availability is limited to governmental entities.

Flights covered all major phases: take off, straight and level flight with constant speed, orbit, straight and level flight with alternating speeds, turns with a maximum bank angle of 60 degrees, pull-ups and push-overs, approaches and landings.

The flights also allowed positioning to be carried out during a wide variety of conditions, such as vibrations, speeds up to 456 km/h, accelerations up to 2 ghorizontal and 0.5–1.5 gvertical, and rapid jerks. The maximum altitude reached during the flights was 3,000 meters.

GPS III Prototype Proves Constellation Compatibility

The Lockheed Martin prototype of the next-generation GPS satellite, the GPS III, has proven that it is backwardly compatible with the existing GPS satellite constellation in orbit.

During tests concluded on October 17, Lockheed Martin’s GPS III testbed successfully communicated via cross-links to Air Force simulators of the current GPS constellation in orbit. The current GPS constellation includes GPS IIR, GPS IIR-M, and GPS IIF satellites.

Testing also demonstrated the ability of an Air Force receiver to track navigation signals transmitted by the GPS III Nonflight Satellite Testbed (GNST). The GNST is a full-sized, functional satellite prototype at Cape Canaveral Air Force Station.

“These tests represent the first time when the GNST’s flight-like hardware has communicated with flight-like hardware from the rest of the GPS constellation and with a navigation receiver,” explained Paul Miller, Lockheed Martin’s director for GPS III Development. “This provides early confidence in the GPS III’s design to bring advanced capabilities to our nation, while also being backward-compatible.”

The first flight-ready GPS III satellite is expected to arrive at Cape Canaveral in 2014, for launch by the Air Force in 2015.
GPS III satellites will be the first GPS space vehicles with a new L1C civil signal designed to make it interoperable with other international global navigation satellite systems.

The GNST has helped to identify and resolve development issues prior to integration and test of the first GPS III flight space vehicle (SV 01). It has gone through the development, test, and production process for the GPS III program first, significantly reducing risk for the flight vehicles, improving production predictability, increasing mission assurance, and lowering overall program costs.

The GPS III team is led by the Global Positioning Systems Directorate at the U.S. Air Force Space and Missile Systems Center.

Lockheed Martin is the GPS III prime contractor, with teammates including ITT Exelis, General Dynamics, Infinity Systems Engineering, Honeywell, ATK, and other subcontractors.

Good News for Users and Manufacturers

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.

Galileo FOC Satellites Endure Simulated Space Tests

The European Space Agency’s newest Galileo satellite has emerged from five weeks of simulated space conditions. On November 29, a hatch slid open to end its thermal-vacuum test, a milestone on the way to orbit.

The satellite was placed in the 4.5-meter-diameter Phenix chamber in ESA’s ESTEC Test Centre in Noordwijk, the Netherlands, in late October. Once inside, the air was pumped out to create a space-quality vacuum. Temperature extremes were also reproduced, with the six copper walls of the thermal tent cooled by liquid nitrogen down to –180°C.

A second Galileo vehicle has been undergoing the same rigors at the site, along with a vibration and shock test to reproduce separation from the launcher. Thermal-vacuum testing on the second model will begin in early 2014. The two satellites will be launched on a Soyuz rocket from Europe’s Spaceport in French Guiana in mid-2014.

The next satellite is expected to arrive at ESTEC in March, with further satellites following every seven weeks or so. A total of 22 FOC satellites are being built by OHB in Germany, with navigation payloads being delivered from Surrey Satellite Technology Ltd. in the UK.

The first Galileo Full Operational Capability satellite emerges from the Phenix test chamber after five weeks of thermal–vacuum testing.

January 1, 2014

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

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

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.
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.
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

December 17, 2013

The System: IRNSS Success, GLONASS Bellyflop

IRNSS Success

The Indian Regional Navigation Satellite System (IRNSS) successfully launched its first satellite on July 1 from the Satish Dhawan Space Centre at Sriharikota spaceport on the Bay of Bengal. An Indian-built Polar Satellite Launch Vehicle PSLV-C22, XL version, carried the 1,425-kg satellite aloft.

IRNSS-1A is the first of seven satellites that will make up the new constellation: four satellites in geosynchronous orbits inclined at 29 degrees, with three more in geostationary orbit. IRNSS-1A is one of the geosynchronous satellites.

Following launch, the master control facility conducted five orbit maneuvers to position the satellite in its circular inclined geosynchronous orbit (IGSO) with an Equator crossing at 55 degrees east longitude. Reports indicate that orbit-raising maneuvers have been completed, and all the spacecraft subsystems have been evaluated and are functioning normally.

IRNSS-1A’s drift eastward from 47 degrees east longitude on July 10 was gradually slowed, and the satellite achieved its assigned inclined geosynchronous orbit, with a 55-degree East equator crossing, by July 18. The orbit inclination is 27.03 degrees.

Payloads. IRNSS-1A carries two types of payloads, navigation and ranging. The navigation payload will operate in L5 band (1176.45 MHz) and S band (2492.028 MHz), using a Rubidium atomic clock. The ranging payload consists of a C-band transponder that facilitates accurate determination of the range of the satellite. IRNSS-1A also carries corner-cube retro-reflectors for laser ranging. Its mission life is 10 years.

GLONASS Bellyflop

A Russian Proton-M rocket carrying three GLONASS navigation satellites crashed soon after liftoff on July 2 from Kazakhstan’s Baikonur cosmodrome. About 10 seconds after takeoff at 02:38 UTC, the rocket swerved, began to correct, but then veered in the opposite direction. It then flew horizontally and started to come apart with its engines in full thrust. Making an arc in the air, the rocket plummeted to Earth and exploded on impact close to another launch pad used for Proton commercial launches.

Despite the loss, GLONASS still has a full operating constellation of 24 satellites.

The crash was broadcast live across Russia. Fears of a possible toxic fuel leak immediately surfaced following the incident, but no such leak has been confirmed. The rocket was initially carrying more than 600 tons of toxic propellants. No casualties or damage to surroundings structures or the town of Baikonur have been reported.

The crashed Proton-M rocket employed a DM-03 booster, which was being used for the first time since December 2010, when another Proton-M rocket with the same booster failed to deliver another three GLONASS satellites into orbit, crashing into the Pacific Ocean 1,500 kilometers from Honolulu.

A Russian government investigation revealed that at least “three of six angular rate sensors [on the booster stage] were installed incorrectly,” to be specific, upside-down. Examination of the wreckage discovered traces of forced, incorrect installation on three sensors. Assembly-line testing at the factory failed to detect the faulty installation.
Several videos of the crash are viewable online (YouTube).

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.

August 1, 2013

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.

July 8, 2013

GPS CNAV Civil Capabilities Now Being Tested

U.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:

Verify and validate the CNAV requirements specified IS-GPS-200F and IS-GPS-705B.
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]

June 17, 2013

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

March 21, 2013

2C or not 2C: An Important Signal Question

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 50^th Space Wing and the 2SOPS (2^nd 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.

January 9, 2013

Tag: CNAV