A ULA Delta IV rocket carrying GPS III SV02 lifts off from Space Launch Complex-37 on Aug. 22. (Photo: ULA)
The U.S. Air Force’s second next-generation GPS III satellite, built by Lockheed Martin, is responding to commands, under control and now using its own internal propulsion system to get to orbit following its successful Aug. 22 launch.
At 11:01 a.m. ET, Air Force and Lockheed Martin engineers at Lockheed Martin’s Launch & Checkout Facility near Denver declared they had full control of GPS III Space Vehicle 02 shortly after the satellite’s separation from its United Launch Alliance (ULA) Delta IV rocket booster. The satellite, nicknamed “Magellan” by the Air Force, began its rocket ride to space with a 09:06 a.m. ET launch from Cape Canaveral Air Force Station.
GPS III SV02 is now climbing towards its operational orbit about 12,550 miles above the earth under the power of its own Liquid Apogee engines. Engineers at Lockheed Martin Space’s Waterton, Colorado facility are commanding the satellite using elements of the GPS Next Generation Operational Control System (OCX) Block 0.
“GPS III SV02 is receiving and responding to commands just as planned. In the days ahead, we’ll finish orbit raising to our operational slot and then send the satellite commands telling it to deploy its solar arrays and antennas,” said Johnathon Caldwell, Lockheed Martin Space’s Vice President for Navigation Systems. “Once we are set up, we’ll begin on-orbit checkout and tests, including extensive signals testing with our advanced navigation payload.”
The payload is provided by L3Harris. The first GPS III satellite launched in December 2018 and its navigation payload has performed beyond expectations on-orbit during pre-operational testing, according to L3Harris.
GPS III SV02 is the second GPS III satellite designed and built by Lockheed Martin to help the Air Force modernize today’s Global Positioning System (GPS) constellation with new technology and capabilities. GPS III satellites provide 3x greater accuracy and up to 8x improved anti-jamming capabilities. GPS III also provides a new L1C civil signal, compatible with other international global navigation satellite systems, like Europe’s Galileo.
The First GPS III Satellite Completes On-Orbit Testing
Image: ULA
GPS III SV02 will be the second GPS III satellite in orbit and the second GPS III satellite now being commanded from Lockheed Martin Space’s facility.
On Dec. 23, 2018, the Air Force launched the first GPS III satellite. Nicknamed “Vespucci,” GPS III SV01 underwent months of checkout and thorough testing of its advanced, new navigation payload provided by Harris Corporation.
“GPS III SV01’s performance exceeded expectations during testing,” Caldwell said. “On July 12, we officially completed all On Orbit Check Out & Test activities. We are excited to see this satellite move to the next phase and perform in an operational environment.”
That’s expected to happen later this year once the first satellite is handed over to the Air Force.
Thinking Ahead From the Ground Up
In preparation for this handover, in 2016, the Air Force awarded Lockheed Martin the GPS III Contingency Operations (COps) contract to upgrade its current GPS ground control system – the Operational Control Segment (OCS) – to be able to fly today’s 31-satellite constellation, as well as the new, more-powerful GPS III satellites, until OCX Block 1, still in development, is delivered.
Lockheed Martin delivered the GPS III COps software upgrade in May and it is currently undergoing preparations for installation.
COps is the latest GPS ground control upgrade project Lockheed Martin has had since it began sustaining the OCS in 2013. In November 2018, the company completed the AEP 7.5 upgrade — the largest architectural change in the system’s history — replacing significant code, hardware and software to improve the system’s cybersecurity capabilities and positioning the Air Force to better operate in contested, degraded and operationally limited environments.
In December 2018, the Air Force awarded Lockheed Martin the GPS Control Segment Sustainment II (GCS II) contract to continue to further modernize and sustain the OCS through 2025.
In 2020, the OCS is expected to receive the M-Code Early Use (MCEU) upgrade, which will allow control of M-Code, an advanced, new signal designed to improve anti-jamming and anti-spoofing, as well as to increase secure access to military GPS signals for U.S. and allied armed forces.
With GPS III SV01 and SV02 now on orbit, GPS III satellites continue to roll off the production line at Lockheed Martin’s advanced $128-million GPS III Processing Facility near Denver. On May 27, the Air Force declared the GPS III SV03 Available for Launch (AFL) and had the company place it into storage waiting for a launch date. GPS III SV04-08 are now in various stages of assembly and test.
In all, Lockheed Martin is under contract to build up to 32 next-generation GPS III/IIIF satellites for the Air Force. Additional IIIF capabilities will begin being added at the 11th satellite. These will include a fully digital navigation payload, a Regional Military Protection capability, an accuracy-enhancing laser retroreflector array, and a Search & Rescue payload.
The Harris-supplied navigation payload before integration into the second GPS III SV. (Photo: L3Harris)
As the U.S. Air Force prepares to launch its second next-generation GPS III satellite, L3Harris Technologies delivered its eighth navigation payload to GPS III satellite prime contractor Lockheed Martin.
The first GPS III satellite launched in December 2018 and its navigation payload has performed beyond expectations on-orbit during pre-operational testing, L3Harris said in a press release.
In May, the Air Force declared the third GPS III satellite Available for Launch, pending a launch date. L3Harris payloads are also already fully integrated in the GPS III 4-6 space vehicles currently in production and testing at Lockheed Martin.
The GPS III navigation payload features a mission data unit (MDU) with a unique 70-percent digital design that links atomic clocks, radiation-hardened processors and powerful transmitters — enabling signals three times more accurate than those on current GPS satellites. The payload also boosts signal power, which increases jamming resistance by eight times and helps extend the satellite’s lifespan.
In 2017, L3Harris announced that it completed development of an even more-powerful, fully digital MDU for the Air Force’s GPS III Follow On (GPS IIIF) program. The new GPS IIIF payload design will further enhance the satellite’s capabilities and performance.
In September 2018, the U.S. Air Force selected Lockheed Martin for a fixed-price-type production contract for up to 22 GPS IIIF satellites. L3Harris is Lockheed Martin’s navigation signal partner for GPS IIIF satellites, and in January received a $243 million award to provide the navigation signals for the first two GPS IIIF satellites, space vehicles 11 and 12.
L3Harris’ expertise in creating and sending GPS signals extends back to the mid-1970s — providing navigation technology for every U.S. GPS satellite ever launched. While the Air Force originally developed GPS for warfighters, millions of people around the world and billions of dollars of commerce now depend on the accurate, reliable signal created and sent by L3Harris navigation technology.
The second GPS III satellite — nicknamed Magellan — is now at the launchpad at Cape Canaveral Air Force Station, Florida, in preparation for liftoff on Aug. 22.
United Launch Alliance tweeted out an image of the encapsulated satellite on its way to the rocket.
The Lockheed Martin-built satellite was originally scheduled for launch on July 25, but the launch was pushed to Aug. 22 because of “an anomaly during component testing at a supplier that created a cross-over concern. Upon further evaluation, additional time is needed to replace and retest the component on the launch vehicle,” ULA said.
Launch Updates
The launch window on Aug. 22 will open at 9 a.m. EDT (1300 UTC) and extend to 9:27 a.m. EDT (1327 UTC), a 27-minute duration. ULA’s live countdown blog begins at 11:45 p.m. EDT (0345 UTC) on Aug. 21. The launch webcast starts at 8:40 a.m. EDT (1240 UTC).
GPS III SV02 will be the 29th and final flight of the Delta IV Medium rocket, the 73rd GPS launch by a ULA or heritage vehicle and marks ULA’s 135th mission.
GPS III SV02 is named Magellan in honor of the Portuguese explorer who led the first expedition to circumnavigate the Earth.
The satellite, encapsulated in the 4-meter-diameter composite payload fairing, was moved overnight last week from its processing facility to the seaside launchpad at a top speed never exceeding 5 mph.
The satellite was hauled by a motorized KAMAG Elevating Platform Transporter (EPT) that provided hydraulic leveling and precision positioning capabilities along the route. The EPT also towed a Portable Environmental Control System (PECS) trailer to supply conditioned air to the payload fairing during the trip.
Once parked in the hoistway on the backside of the Mobile Service Tower (MST), technicians used the crane system in the gantry the next morning to carefully lift the satellite onto the Delta IV rocket’s second stage to complete a successful vertical integration of the launch vehicle and payload. The fully assembled rocket now stands 207 feet tall.
A tip-to-tail electrical test of the combined payload and launch vehicle will occur next, an operation known as the Integrated Systems Test (IST). Once that is completed, the comprehensive process to verify flight readiness will begin in parallel to final vehicle closeouts for the launch targeted for Aug. 22 at 9 a.m. EDT (1300 UTC).
ULA rockets have successfully launched 70 GPS satellites since 1978.
ULA technicians transport the GPS III satellite to the Delta IV launchpad. (Photo: United Launch Alliance)The Delta IV rocket leaves the the Horizontal Integration Facility (HIF) aboard a 36-wheel, diesel-powered transporter on May 28 and traveled to Space Launch Complex-37. The trip took 40 minutes. (Photo: ULA)
The second next-generation GPS III satellite — nicknamed “Magellan” by the U.S. Air Force — is encapsulated and ready for its planned July 25 launch.
On June 26, Lockheed Martin Space and United Launch Alliance (ULA) technicians completed encapsulating GPS III Space Vehicle 02 (GPS III SV02) in its launch fairings at the company’s Astrotech Space Operations facility, where the satellite has undergone pre-launch processing and fueling since its March 19 arrival in Florida. This final step enclosed GPS III SV02 in a protective, aerodynamic, nose-cone shell.
In the coming days, the enclosed GPS III SV02 satellite will be mounted to a ULA Delta IV rocket for launch. The current window for launch on July 25 opens at 10:55 a.m. ET.
“GPS III SV02 is launching just a brisk seven months after the nation’s first GPS III satellite lifted off back in December. The first satellite’s performance during on-orbit testing has exceeded expectations,” said Johnathon Caldwell, Lockheed Martin’s vice president for Navigation Systems. “We are excited to deploy more GPS III satellites so this new technology and capabilities can be distributed constellation-wide.”
GPS III satellite production and launch cadence is picking up. On May 27, the Air Force declared the next GPS III satellite, GPS III SV03, available for launch, pending an official launch date.
“More GPS III satellites are coming. If you looked at our production line back in Denver today you would see GPS III space vehicles 04, 05 and 06 already fully-assembled and in various stages of testing. And space vehicles 07 and 08 are being built up at the component assembly level now,” Caldwell added. “It is a smooth, efficient, methodical process.”
Lockheed Martin is under contract to develop and build up to 32 GPS III/IIIF satellites for the Air Force. GPS III will deliver three times better accuracy and provide up to eight times improved anti-jamming capabilities. GPS III’s new L1C civil signal will make it the first GPS satellite to be interoperable with other international global navigation satellite systems, like Galileo.
Additional GPS IIIF capabilities will begin being added with the 11th satellite. These will include a fully-digital navigation payload, a Regional Military Protection capability, an accuracy-enhancing laser retroreflector array, and a search-and-rescue payload.
The Harris-supplied navigation payload before integration into the second GPS III SV. (Photo: Harris)
L3Harris Technologies announced the successful completion of the all-stock merger between Harris Corporation and L3 Technologies on June 29. Headquartered in Melbourne, Florida, L3Harris becomes the sixth largest defense company in the U.S., and a top 10 defense company worldwide, with approximately $17 billion in revenue and 50,000 employees, including 20,000 engineers and scientists.
Both companies have long been dominant presences in the U.S. GPS industry: Harris as a provider of the GPS satellite navigation payloads and geospatial intelligence software products, and L3 as a provider of military GPS user equipment and guided munitions. Both companies supply a wide range of other geospatially-related products as well.
L3Harris has organized its operating businesses into four segments to best meet customers’ mission requirements and leverage the combined company’s broad technical capabilities:
Integrated Mission Systems — headquartered in Palm Bay, Florida, with approximately $4.9 billion in revenue. Includes intelligence, surveillance and reconnaissance; advanced electro optical and infrared solutions; and maritime power and navigation
Space and Airborne Systems — headquartered in Palm Bay, Florida, with approximately $4.0 billion in revenue. Includes space payloads, sensors and full-mission solutions; classified intelligence and cyber defense; avionics; and electronic warfare
Communication Systems — headquartered in Rochester, New York, with approximately $3.8 billion in revenue. Includes tactical communications; broadband communications; night vision; and public safety
Aviation Systems — headquartered in Arlington, Texas, with approximately $3.8 billion in revenue. Includes defense aviation products; security, detection and other commercial aviation products; air traffic management; and commercial and military pilot training
Shares of Harris common stock, which traded on the NYSE under the ticker symbol “HRS,” began trading on July 2 under the ticker symbol “LHX.” L3 Technologies shares ceased trading upon market close on June 28 and have converted into 1.3 L3Harris shares for each L3 share.
The merger comes at approximately the same time that two other leading GPS companies, Raytheon and United Technologies, itself a merger including the former Rockwell Collins, now Collins Aerospace, also merged.
Technicians successfully integrated the U.S. Air Force’s third GPS III space vehicle (GPS III SV03) on August 14, 2017. (Photo: Lockheed Martin)
On May 22, Lockheed Martin delivered the GPS III Contingency Operations (COps) software upgrade to the U.S. Air Force’s current GPS ground control system.
The upgrade will enable the Air Force to start commanding the new, next-generation GPS III satellites now coming off the production line and beginning to launch.
And the new GPS III satellites are coming. The first GPS III satellite launched in December 2018; the second GPS III shipped to Cape Canaveral in March for a July launch; and on May 17, the Air Force declared the third new GPS III “Available for Launch” next.
Ground System. The challenge was modernizing the current ground system — formally known as the GPS Architecture Evolution Plan Operational Control System (AEP OCS) — to fly the legacy constellation, as well as the new, modern GPS III satellites, until the next generation Operational Control System (OCX) Block 1, still in development, is delivered.
To address this, in 2016, the Air Force contracted Lockheed Martin to develop the GPS III COps program. Currently, the AEP OCS controls 31 GPS IIA, IIR, IIR-M and IIF satellites launched between 1993-2016. With the AEP OCS’ new GPS III COps upgrade, the Air Force will be able to command and control both the legacy satellites, as well the more powerful GPS III satellites.
Lockheed Martin shipped the U.S. Air Force’s first GPS III to Cape Canaveral, Florida ahead of its expected July launch. (Photo: Lockheed Martin)
“Positioning, Navigation and Timing is a critical mission for our nation and COps will allow the Air Force to gain early access to its new GPS III satellites,” said Johnathon Caldwell, Lockheed Martin’s vice president for Navigation Systems. “We just finished Final Qualification Testing and delivery on COps, and it will be integrated and installed on the AEP OCS over the summer. We look forward to the Air Force ‘flying’ a GPS constellation on the COps OCS which includes the new GPS III satellites, later this year.”
Meanwhile, the first GPS III space vehicle (GPS III SV01), launched in December 2018, is finishing up pre-operational on-orbit check-out. It continues to be controlled by OCX Block 0 software installed at Lockheed Martin’s GPS III Launch and Checkout Center at the company’s Denver facility. GPS III SV01 is expected to be “handed over” to the COps OCS later this year after the legacy constellation is moved over to the updated AEP OCS.
Lockheed Martin has sustained the AEP OCS since 2013. In November 2018, the company completed the AEP 7.5 upgrade — the largest architectural change in the systems history — replacing significant code, hardware and software to improve the system’s cybersecurity capabilities and positioning the Air Force to better operate in contested, degraded and operationally limited environments.
In December 2018, the Air Force awarded Lockheed Martin the GPS Control Segment Sustainment II (GCS II) contract to continue to further modernize and sustain the AEP OCS through 2025. In 2020, the AEP OCS is expected to receive the M-Code Early Use (MCEU) upgrade, which will allow control of M-code, an advanced, new signal designed to improve anti-jamming and anti-spoofing, as well as to increase secure access to military GPS signals for U.S. and allied armed forces.
Lockheed Martin is under contract to develop and build up to 32 GPS III/IIIF satellites. GPS III will deliver three times better accuracy and provide up to eight times improved anti-jamming capabilities. GPS III’s new L1C civil signal will make it the first GPS satellite to be interoperable with other international global navigation satellite systems. Additional “IIIF” capabilities, beginning at the 11th satellite, will include a fully digital navigation payload, Regional Military Protection, an accuracy-enhancing laser retroreflector array, and a Search & Rescue payload.
Raytheon Company’s GPS Next-Generation Operational Control System program, known as GPS OCX, completed final qualification testing of the system’s modernized monitor station receivers, which are now ready to be installed around the world starting in August.
GPS OCX is the enhanced ground control segment of a U.S. Air Force-led effort to modernize America’s GPS system.
“The modernized receivers give GPS OCX the ability to receive and decrypt all GPS III military and civil signals, a critical capability the current system doesn’t have,” said Dave Wajsgras, president of Raytheon Intelligence, Information and Services. “Monitor station receiver installation keeps us on track for full system delivery by our June 2021 contractual deadline.”
The modernized receivers will measure and monitor legacy military and civilian signals sent by the current GPS satellite constellation plus the new signals sent by the next-generation GPS III.
The receivers will also feed correction models at the master control station, giving U.S. Air Force satellite controllers the information necessary to make key adjustments to maximize accuracy.
Feature photo: GPS ground antenna at Schriever Air Force Base, home of the 50th Space Wing. (Photo: Raytheon)
Harris Corporation has received a $243 million contract from Lockheed Martin to provide fully digital navigation signals for the first two GPS III Follow-On (GPS IIIF) satellites — to deliver stronger signals, with greater operational flexibility.
Harris’ GPS IIIF fully-digital Mission Data Unit (MDU), the heart of the satellite’s navigation payload which generates the GPS signals, will provide more powerful signals, assure flawless clock operations for GPS users, and add flexibility to adapt to advances in GPS technology, as well as future changes in mission needs.
It will provide improved capabilities over Harris’ 70-percent-digital MDU used for GPS III Space Vehicles 01-10 (GPS III SV01-10).
The new MDU also offers the Air Force a smooth transition to its GPS OCX ground control segment. Harris will seamlessly port its digital signal design, minimizing both integration risks and associated costs.
In September 2018, the U.S. Air Force selected Lockheed Martin, with Harris as its navigation signal partner, to build up to 22 GPS IIIF satellites, with a total estimated contract value up to $7.2 billion.
The Air Force expects the first GPS IIIF satellite, SV11, to be available for launch in 2026.
Launched aboard GPS III SV01 in December 2018, Harris’ first GPS III navigation payload began broadcasting navigation signals on January 8. While testing of the first-of-its-kind satellite continues, the payload has performed beyond expectations.
Harris has provided navigation technology for every U.S. GPS satellite ever launched, enabling the reliable GPS signal that millions of people — including U.S. soldiers — and billions of dollars in commerce depend on every day.
Photo: U.S. Air Force / Staff Sgt. Scott H. Spitzer
Much development has been necessary to enable the new M-code capability on more than 700 weapon systems that require it. This article overviews M-code, the updates to antenna and receiver technology to make these varied platforms M-code ready, and perspectives from key stakeholders in the M-code community.
December 23, 2018, marked an important milestone for GPS. The successful launch of satellite USA-289 represented a key success in what has been a monumentally expensive government program, beset by delays and overspends.
The launch of the first GPS Block III satellite, the first that can provide the full military M-code capability, effectively commenced the physical roll-out of modern M-code hardware.
Ground Control. As far as the space segment is concerned, M-code is finally underway. What about the ground segment? The next-generation GPS operational control system, GPS OCX, is essential for use of the full capabilities of the new Block III satellites. It has been under development for some time.
OCX has drawn Congressional criticism and correlative media attention, but recent reports have been more positive. Since the Nunn-McCurdy breach of 2016, when the project’s future hung in the balance, accounts have grown gradually optimistic. Budget and schedule were re-baselined, and contractor Raytheon’s corrective actions generated results. In the fall of 2017 the Air Force took delivery of OCX Block 0, marking a significant milestone. Block 0, also known as the Launch and Checkout System (LCS), demonstrated compliance with contractual requirements and was accepted by the Air Force.
In spring 2018, Block 0 underwent a series of cybersecurity tests and passed, validating the security architecture of the system. All this puts Raytheon on track to deliver OCX Block 1 in 2021, providing full operational capability. Block 1 and Block 2 are intended to be delivered together, adding operational control of the modernized satellites and signals, including L1C and the modernized M-code.
“There have been no schedule slips with the GPS OCX program since 2017, and the GPS III launch last December was clear proof of our progress,” stated Dave Wajsgras, president of Raytheon’s Intelligence, Information and Services business. “We will continue to meet all of our commitments, and importantly, we will meet our June 2021 contractual deadline.”
Col. Steve Whitney of the GPS Directorate wrote in this magazine in December 2018 that “The journey over the past few years has been challenging, but we have emerged stronger, armed with better metrics, and a culture of integrated development (often called DevOps) which puts us on a path to success. There will be challenges and risks in the path ahead but rather than mountains to climb, I see these more as standard blocking and tackling of a software-intensive program.”
Meanwhile. The Air Force plans to deploy M-code capability in 2020, and OCX seems unlikely to be ready. For this reason, Lockheed Martin was awarded a contract to modernize the existing ground infrastructure as a “gap filler.”
The GPS Control Segment Sustainment II (GCS II) contract was awarded on Dec. 21, 2018, and is worth $462 million. GCS II will support operational capability of M-code in 2020, and continues until 2025, and so there will be a period of overlap between GCS II and OCX, essentially providing two options for controlling the new GPS III constellation. In one view, the Air Force is backing two horses to improve chance of winning: OCX the preferred solution, with GCS II almost like an insurance policy.
With the GPS III ground and space segments looking relatively healthy, attention turns again to the user segment.
WHY M-CODE?
Until now, the military has used the classic P(Y) signal: a binary phase shift keying (BPSK)-modulated encrypted wideband signal. It offers both greater accuracy and increased jamming resistance when compared to the civilian C/A code still employed by the vast majority of GPS receivers.
But the P(Y) code has its drawbacks in the modern world: its wide main lobe sits directly over the top of the C/A code signal (see Figure 1), essentially occupying the same spectrum. When the civilian C/A signal is jammed, the military P(Y) signal is at the very least degraded, if not also jammed itself. It also uses a relatively simple encryption scheme that does not meet today’s cyber security requirements.
Figure 1. C/A, P(Y), and M-Code signal power spectra. (Graphics: Mike Jones)
The M-code signal, on the other hand, is the first military GPS signal to use the BOC modulation scheme. BOC modulation gives signals their distinctive two-lobe appearance, spreading the signal’s energy away from the band center.
The wide spacing of the two sidebands separates the M-code signal from the civilian signals (the legacy C/A signal or the new L1C signal on the L1 frequency, and the L2C signal on the L2 frequency).
Amongst other things, this allows the military to jam the civilian codes without noticeably degrading the M-code signal. Often referred to as blue force electronic attack (BFEA), this is essentially a new facet to navigation warfare (NAVWAR), where enemy use of GPS can be denied whilst allowing friendly forces to continue using it.
The wider occupied bandwidth and increased signal power also help to make M-code more resistant to jamming. M-code also makes use of more modern and flexible encryption methods, ensuring it will be secure and safer from threats such as spoofing attacks.
Scepticism. Defense programs are known for their long procurement cycles, but even by these standards, M-code has taken an extremely long time to get where it is today. Given the enormous cost of the program, and the fact that there is still, as yet, no operational benefit to show from it, many people have questioned its worth. At the time it was conceived it represented a dramatic step forward in military capability but, because it has been so long in development, its operational benefit is becoming diluted.
When M-code was conceived, GPS was still the only operational GNSS in town: everybody had to use GPS — or nothing. Today, the picture differs greatly. During M-code’s insanely slow progress, other GNSS systems have come along, offering their own encrypted signals of a similar ilk. Looking at Figure 2, M-code no longer appears as special as it once was. Its BOC(10,5) signal sits inside the main lobes of Europe’s Galileo PRS signal, which uses a BOC(15,2.5) scheme, and China’s Beidou B1A signal using BOC(14,2).
Figure 2. GNSS encrypted signals around the L1 frequency. (Graphics: Mike Jones)
If you were China, you might consider jamming the central 24 MHz of the L1 band, taking out M-code, whilst still having an operational military service for yourself. Or if you were Russia, you might jam 34 MHz of bandwidth, taking out the US, Chinese, and European systems, whilst still having your GLONASS L1SC military service to use. The situation is more complex than that, of course: each service has the potential to increase signal power in times of conflict, and there is more than one frequency that can be used. But it does demonstrate the essence of the problem: The modern battlespace has moved on, and M-code hasn’t.
CHALLENGES OF RECEIVER DESIGN
Figure 3. C/A code ACF.
With complex signals come complex receivers, and there several headaches when it comes to M-code receiver design. The first is the nature of the BOC signal itself, which has a complex correlation function. Consider Figure 3, which shows the autocorrelation function (ACF) of the traditional civilian C/A code signal. The single peak of the function makes acquisition and tracking a simple process; traditionally early, prompt and late (E,P,L) correlator arms can be used in the tracking process.
Figure 4. L1Cd ACF.
The newer BOC-type signals have a more complex ACF. Figure 4 shows the ACF of the new L1Cd civilian GPS signal, which uses a form of BOS(1,1) modulation. In addition to the main lobe, there are now two side lobes. Receivers must be careful not to lock on to one of the side lobes instead of the main lobe: the receiver architecture starts to become a little more complex.
Figure 5. M-code ACF.
Now consider the ACF of the M-code signal, shown in Figure 5. Like other high-order BOC-type signals, M-code exhibits multiple lobes in the ACF, making robust acquisition and tracking a far more troublesome process. Furthermore, the high bandwidths require high sample rates, which lead to higher power consumption in the hardware.
Another major headache associated with M-code receivers is, of course, the encryption process. Not because encryption is difficult, but again because of the power consumption implications. Consider that each GPS receiver needs to run an encryption engine instance, for each satellite it might wish to receive. Running a high-grade encryption algorithm at a high chipping rate, for a dozen satellites, is a power-consuming process. For dismounted soldiers with limited battery capacity, this is a big deal.
Some people argue that the high-grade encryption process for M-code is too complex. Consider why we want to encrypt a GNSS signal in the first place: firstly to prevent someone from spoofing our signal, and secondly to prevent unauthorised users from using the service. Given that the encryption keys are rolled regularly, how much does it matter if an adversary manages to compromise the encryption? This isn’t a communications security problem: we are not talking about loss of classified information, so there’s an argument that a simpler, less power-hungry form of encryption might have been used instead.
ANTI-JAM ANTENNA COMPATIBILITY
Although M-code offers a certain level of jamming resistance, it is still vulnerable to attacks. As a signal it might have a bit more power, and a bit more bandwidth, than some other signals. But it is, after all, still a GNSS signal, and it can be jammed by an adversary. Where an operational threat analysis indicates that an increased level of jamming resistance is required, then M-code receivers need to be integrated with anti-jam antennas.
Anti-jam antennas, usually referred to in the GNSS community as controlled reception pattern antennas (CRPAs), have been the anti-jam tool of choice for several decades now. I overviewed these in an April 2017 newsletter column. CRPA manufacturers have had to ensure that their products are “M-code ready,” such that they can be seamlessly attached to M-code receivers as and when they appear.
This hasn’t been a recent process: as far back as 2002, the GAS-1 antenna (Raytheon) underwent a series of qualification tests to ensure compliance with M-code. Around 2005, the ADAP antenna (also Raytheon) was launched with a host of M-code features — again an illustration of just how slow the M-code program has moved, given that other technology has been “M-code ready” for 10 or 15 years already.
What’s involved in making a CRPA M-code compatible? Firstly the increased bandwidth: the antenna electronics must digitize the wider bandwidths. Along with the wider bandwidth comes new filtering shapes to ensure optimum performance.
Space-time adaptive processing (STAP) and space-frequency adaptive processing (SFAP) techniques potentially require more taps to ensure high null depths can be maintained across the full bandwidth. The increased power of the M-code signal, particularly if features like spot beam are used, presents another complication to CRPAs: they must not treat the high-power satellite signals as jammers, and try to remove them.
Testing CRPAs presents a challenge to manufacturers: how do you prove that your antenna doesn’t corrupt the M-code signal, when there’s no M-code signal to test it with? To work around this issue, pseudo M-code signals have been used for testing, where representative BOC(10,5) signals without the real encryption are passed through the CRPA and examined for distortion.
RECEIVER DEVELOPMENT STATUS
Due to the security considerations surrounding M-code, only three US organizations are authorized to produce modules: Collins Aerospace, Raytheon and L3. Here are the answers from Collins Aerospace and L3, the answers from Raytheon will appear in later issue.
What are the technical challenges associated with developing an M-code receiver?
Collins Aerospace. The Collins Aerospace Modernized GPS User Equipment (MGUE) Increment 1 development like the SAASM PPS receiver developments faced very challenging technical requirements to support our war fighter needs in an ever-evolving threat environment. Like other complex developments the challenges are initially technical and then transition to integration/test and certification. On the technical front optimizing receiver performance balanced against power consumption are always at the forefront. In addition, it is important to maximize backwards compatibility so as to minimize downstream integration costs while adding an entirely new signal that runs in parallel to the existing system. Collins Aerospace is pleased with the technical development and are actively supporting the integration with both receivers and technical support.
To date, we have delivered more than 770 MGUE receivers to the Air Force to support Air Force, lead platform and DoD-wide Integration and test. Soon the total will grow to nearly 1,100 receivers to support expanded integration and test following the completion of Collins Aerospace security certification.
L3. M-code GPS User Equipment (MGUE) technologies exist today.L3’s Ground Based GPS Receiver Application Module – Modernized (GB-GRAM-M) is a fully-functioning unit that is currently baselined and undergoing an independent Technical Requirements Verification (TRV) by the GPS Directorate.During TRV, each requirement from the Technical Requirements Document (TRD) is independently evaluated for compliance. Upon completion of the TRV, the design is baselined with complete documentation enabling platforms and prime equipment to integrate from a known baseline with low risk. Following integration, operational testing can start immediately to support fielding when M-Code Early Use (MCEU) becomes operational. The TRV of L3’s GB-GRAM-M is planned to be completed by the second quarter of 2019.
L3 resolved numerous technical challenges in developing M-code GPS technologies. The first and ever-present challenge is changing and evolving requirements. Most of these requirement changes are in response to evolving threats that have driven changes into the GPS receiver and/or to higher-level systems. Asan example, the U.S. Army’s Assured PNT (A-PNT) is implementing M- code GPS along with external sensors to establish and maintain an assured solution even in GPS-challenged environments. Other challenging requirements include meeting the security requirements, implementing and testing anti-spoofing algorithms, and ensuring backward compatibility with legacy receivers.
What are the intended platforms for your MGUE?
Collins Aerospace. The Collins Aerospace MGUE receivers are intended to support all warfighter domains: ground, airborne, maritime and munitions to support compliance with Public Law 111-383 SEC. 913 issued in Fiscal Year 2011. Per this directive, M-code is intended for all DoD applications with the exception of passenger vehicles or commercial vehicles with GPS installed. Now that the satellite and control segments of the capability are coming on line, we are working diligently to ensure that user equipment is available for all domains.
L3. L3 has products to meet current market demand. Under the MGUE program, L3 developed a GB-GRAM-M, which is a standard Modular Open Systems Architecture (MOSA) design. The GB-GRAM-M is designed to fulfill retrofit replacements of SAASM receivers, as well as being a primary component of A-PNT systems. L3’s M2GRAM ASIC is the core of our receiver, a GPS module that incorporates signal processing, cryptography, and positioning, velocity, and timing (PVT) processing. The M2GRAM ASIC is capable of being implemented in other form factors for applications beyond ground-based applications. As an example, the M2GRAM is implemented in a GPS receiver specifically designed for Precision Guided Munitions (PGM) applications and was used in a gun launched, guide-to-target demonstration operating as a PGM receiver.
L3 is also augmenting the GPS receiver through the integration of several other technologies, including controlled reception pattern antennas with digital antenna electronics, inertial systems and external sensors, and GPS-denied capabilities. M-code technologies are being implemented in Mounted A-PNT Systems (MAPS), Dismounted A-PNT Systems (DAPS), and handheld systems to bring capabilities to the warfighter.
What is the expected timeline for your MGUE development, acceptance testing, and delivery?
Defense Advanced GPs Receiver (DAGR) from Collins Aerospace, equipping infantry and other warfighters. (Photo: Collins Aerospace)
Collins Aerospace. The Collins Aerospace receivers are supporting ongoing DoD integration and test and our MGUE Increment 1 program is aligned with the Air Force GPS Enterprise roadmap. Ultimately, the Department of Defense (DoD) M-code programs will set the production delivery schedules.
We anticipate that the M-code production ramp-up and continued SAASM PPS receiver production will have a production overlap. Our Collins Aerospace in-house PPS GPS receiver manufacturing capability is ready to support the DoD demand for both M-code and SAASM. Collins Aerospace is fully committed to manufacturing Increment 1 M-code receivers to meet the warfighter’s needs across Airborne, Weapons and Ground, we know the transition from SAASM to M-code will take years. Therefore, Collins Aerospace will continue to manufacture SAASM receivers for years to come as the International MOD Policy for M-code use is still being formulated.
L3. L3’s GB-GRAM-M is now available. L3 received security certification and approval in 2016 and TRV is planned for completion in the second quarter of 2019. With TRV, L3 is receiving a new security certification and approval of the latest receiver update. Government agencies, prime contractors and laboratories can order GB-GRAM-M now with delivery in the fourth quarter of 2019.
What does testing and verification process involve?
Collins Aerospace. As with any Precise Positioning Service (PPS) GPS development, the testing involves functional verification of the receiver in a wide variety challenging of environmental, thermal, electromagnetic interference/ high-intensity radiated field (EMI/HIRF) environments. Collins Aerospace is leveraging proven test and verification approaches founded upon our long history of successful product introductions and field performance. As this is a PPS receiver it is also essential the receiver design comply with the government’s required Security Approval process.
L3. The testing and verification of L3’s GB-GRAM-M included internal testing and independent testing through the GPS Directorate’s TRV process. Further risk reduction testing within the MGUE program is planned as Phase IV testing where the GB-GRAM-M is integrated into a lead platform for the U.S. Army and a lead platform for the U. S. Marine Corps. An operational assessment is performed on both lead platforms to assure common problems associated with integration and operational testing are addressed prior to implementing M-Code GPS Receivers across all of the platforms.
Will the MGUE be compatible with CRPA anti-jam antennas; are there any special considerations for this?
Collins Aerospace. The Collins Aerospace product family includes our Digital Integrated Anti Jam Receiver (DIGAR) product family that leverages CRPA anti-jam antennas for enhanced anti-jam (AJ) performance. Our DIGAR AJ technology enhances the performance with fixed reception pattern antenna (FRPA), CRPA and is compatible with all PPS waveforms. Regarding the interfaces between the receiver and the anti-jam antenna electronics, a GPS receiver with a standard RF interface is compatible with a CRPA in nulling mode and FRPA antennas. Advanced capabilities such as beamforming/beamsteering require tight coordination and additional interface with the GPS receiver.
L3. The GB-GRAM-M is designed to operate with a fixed reception pattern antenna (FRPA). A CRPA antenna using digital antenna electronics to generate signals matching the characteristics of a FRPA is fully compatible with the GB-GRAM-M. With a higher level of integration of a GPS receiver and a CRPA, the system capabilities are greatly enhanced. L3 has performed this integration and can perform advanced capabilities such as angle of arrival and beamforming using M2GRAM, digital antenna electronics, and CRPA technologies. These capabilities can be found in L3’s Mounted Assured PNT System (MAPS) and Anti-Jam Antenna System (AJAS) products.
Army Stryker ground combat vehicle. (Photo: Karolis Kavolelis / Shutterstock.com)
OPERATIONAL DEPLOYMENT
The U.S. Air Force GPS Directorate provided answers to the following questions regarding MGUE.
Which platforms will be equipped with M-code-capable MGUE, and how many of each?
GPS Directorate. The Air Force is developing M-code-capable GPS receivers under the MGUE Increment 1 program. The receivers in development will be provided to four service-specific lead platforms for integration, developmental, and operational testing. Lead platforms are:
the Army Stryker ground combat vehicle,
the Air Force B-2 Spirit bomber,
the Marine Corps Joint Light Tactical Vehicle (JLTV),
and the Navy Arleigh-Burke class destroyer (DDG).
Following the lead platform efforts, procurement of M-code-capable GPS receivers will be decided by the Services and executed by individual platforms and programs.
What are the timelines for rolling out M-code on these platforms?
GPS Directorate. Early integration and test activities have already begun for each MGUE lead platform. Operational testing is expected to begin in 2020 and complete in 2021, which is a key activity to enable the fielding of M-code-capable systems.
B-2 Spirit multi-role bomber capable of delivering both conventional and nuclear munitions. In December 2017, the Air Force completed a series of successful flight tests of M-code GPS using a Raytheon Company receiver on board a B-2 Spirit at Edwards Air Force Base, California. (Photo: U.S. Air Force/Bobby Garcia)
What advantages will M-code bring, over existing military GPS receivers?
GPS Directorate. Modernized GPS receiver cards under development with the Air Force MGUE Increment 1 program will enable the use of M-code and provide U.S. forces with enhanced position, navigation, and timing capabilities, in addition to improving resistance to threats, such as jamming efforts by adversaries.
How will keys and key distribution be managed?
GPS Directorate. None of this is publically releasable.
Will M-code be made available to other friendly nations? If so, how is this managed?
GPS Directorate. The current policy allows for the sale of M-code equipment to all 57 authorized GPS PPS nations. The M-code technology will be made available to these nations through the Foreign Military Sales process.
USER PERSPECTIVE
The Department of Defense supplied answers to the following questions for users and warfighters.
What are the benefits you perceive will come from new M-code GPS equipment?
DoD. Provides U.S. forces with enhanced position, navigation, and timing capabilities, in addition to improving resistance to threats, such as jamming efforts by adversaries.
Will it change how you perform military operations, or enable any new ones?
DoD. Modernized GPS receivers provide the next-generation GPS capabilities to the warfighter. Operational testing will enable the services to determine operational utility of MGUE. It will ensure our soldiers, sailors, airmen, and marines have the ability to get in, accomplish their mission, and get home accurately.
How will M-code-based GPS receivers be brought into operational service? Will there be a mass upgrade of assets, or a phased introduction?
DoD. Procurement of M-code-capable GPS receivers will be decided by the Services and executed by individual platforms and programs.
When managed by a new ground control system, GPS III satellites will offer triple the accuracy and eight times the anti-jamming capabilities of the satellites currently comprising the U.S. Air Force’s GPS constellation. Users military and civilian will reap ample benefits.
Everything changed for space-based positioning, navigation and timing around the world on Dec. 23, 2018. Or maybe it didn’t. The innovations heralded by the launch of the first GPS III satellite will take years more to occur. We tabulate here the advances that Generation Three will bring over GPS-to-date, and review the timeline for their actual arrival.
While these new capabilities exist — in concept — in space, they can’t be leveraged on the ground (or in the air, or at sea) until a sufficient number of additional GPS III satellites have joined the constellation, and until a new ground control system comes online. This will occur — perhaps — in 2023. At that time the satellites’ talents will be unleashed.
“As more GPS III satellites join the constellation, it will bring better service at a lower cost to a technology that is now fully woven into the fabric of any modern civilization,” stated Lt. Gen. John Thompson, commander of the U.S. Air Force’s Space and Missile Systems Center and the Air Force’s program executive officer for space.
The many GPS III upgrades should make the service more reliable and accurate for civilians, more secure against those who want to jam military users, and more cyber-secure for everyone.
TALKIN’ ‘BOUT OUR GENERATION
GPS constellations have grown through six major iterations since 1978. The sixth, GPS IIF, rose during the years 2010 to 2016. Those 12 satellites are all designed to last 12 years. Some of their notable features include the ability to receive software uploads, better jamming resistance and increased accuracy.
GPS III, the seventh generation, will launch nine more satellites to join SV01 already in space. GPS III SV02 is scheduled to launch in July of this year, SV03 in late 2019, and SV04 in 2020. The final III payload should rise in 2023. From that point on, the follow-on era of GPS IIIF takes over.
How Long, How Long? “Projections for how long the current constellation will [continue to] be fully capable have increased by nearly two years to June 2021, affording some buffer to offset any additional satellite delays,” reported the Government Accounting Office at the end of 2017. This provided some schedule buffer for launching the first GPS III satellite, but it did not reduce the desire to launch as soon as the booster rocket became available.
The new birds will introduce new capabilities to meet higher demands of both military and civilian users: once filled out, the GPS III constellation will bring three times better accuracy and up to eight times improved anti-jamming capabilities. Spacecraft life requirement will extend to 15 years, 25 percent longer than the latest GPS satellites and twice the original design life of the oldest satellites on orbit today.
The new L1C civil signal broadcast by GPS III is an interoperable signal with other international global navigation satellite systems, like Galileo, improving connectivity for civilian users.
GPS III will eventually actualize full M-code capability — carried aboard the IIR-Ms and IIFs but not yet completely implemented — in support of warfighter operations. GPS III M-code capability exceeds that of GPS IIR-M and GPS IIF.
GPS III will complete the deployment of the L2C civil signal and the L5 safety-of-life signal capabilities that began with \GPS IIR-M and GPS IIF satellites.
Finally, GPS III will enact improved integrity: the ability of the satellite to detect and issue alerts on its own reduced accuracy, should that phenomenon ever occur.
Military Signal Power Up. Encrypted M-code signals will be up to eight times more powerful than currently. This makes them more reliable. but also enables the sats to overcome efforts to jam their signals.
Other signals also offer increased signal power at the Earth’s surface. L1 and L2: −158.5 dBW for aC/A code signal and −161.5 dBW for the P(Y) code signal. L5 will be −154 dBW.
Family Features. The most recent generations of the GPS constellation. IIR, IIR-M and III were produced by Lockheed Martin, while IIF was built by Boeing. One GPS IIA satellite is still in operation, at 25 years young (design life was 7.5 years). All satellites carry Harris Corporation payloads. (Graphic sourced from: Lockheed Martin and Boeing Co.)
L SIGNALS
L2C, the second open GPS signal, after L1 C/A, has been available from every new GPS satellite since the first IIR-M launch in 2005. L5, the third open GPS signal, became available with the first IIF launch in 2010. Now L1C, the fourth open GPS signal, joins the band, broadcasting from every new GPS satellite, starting with the recent GPS III launch (see First Light).
The first GPS III satellite is in checkout and testing that could last up to 18 months before it enters service. “After its Dec. 23 launch, GPS III SV01 successfully completed its orbit raising and deployment of all of its antennas and solar arrays. On Jan. 8, the satellite’s navigation payload began broadcasting navigation signals,” said Johnathon Caldwell, Lockheed Martin vice president for navigation systems. “On-orbit testing continues, but the navigation payload’s capabilities have exceeded expectations and the satellite is operating completely healthy.”
Testing, Testing. Using the Air Force’s Back-to-Basics program, which involved early prototyping and simulations, Lockheed Martin developed GPS III with an approach that involved rigorous quality-build certificates, component testing and system-level testing. The comprehensive requirements verification and validation process ensured more than 30,000 requirements were achieved. The system functional qualification includes the performance verification in multiple environmental tests, including the acoustic, thermal vacuum (TVAC) and electromagnetic spectrum.
“We consider thermal vacuum the gold standard for testing any satellite before it goes into operations,” Col. Steve Whitney, director, GPS Directorate, wrote in GPS World in December. “It really is putting the craft through the paces. When it goes through the testing, the satellite is on. It is working. It is exposing it to the heat and the cold and the zero pressure while the satellite is functional. The entire thermal vac testing from start to end is about 70 days. Test like you fly. From the time it launches and deployment sequence, we test it like it is real. Minus the shaking, the satellite thinks it is getting launched. Meanwhile, our people are looking at the data and its health. TVAC is a huge milestone for a satellite to go through and come out no issues.”
To date, more than 90 percent of parts and materials for all 10 GPS III satellites have been received from more than 250 aerospace companies in 29 states.
BRAIN OF THE BUNCH
THE FIRST GPS III satellite was fully assembled and entered into SV single-line flow when Lockheed Martin technicians integrated its system module, propulsion core and antenna deck. (Photo: Lockheed Martin)
Harris Corporation is a subcontractor to Lockheed Martin for development and production of GPS III Mission Data Units (MDUs) and transmitters for the GPS space section. Six have been delivered.
The Harris MDU, together with the Atomic Frequency Standards and the L-band transmitter equipment, make up the Navigation Payload Element. The MDU performs the primary mission of the GPS satellite: generation of the navigation signals and data on a continuous basis. The MDU controls the generation of the precise timing signals used for navigation signals while distributing the timing signals to other satellite components.
This MDU is 70 percent digital. The next to come, aboard GPS IIIF satellites, will be fully digital.
When asked about the advantages of an all-digital payload, Harris Corporation’s Jason Hendrix, PNT program director, told GPS World in April 2018, “The advantages and the 30 percent difference are the timekeeping system portion. We’re moving from manual, analog timing to digital to deliver to the Air Force more flexibility. It’s a nice option to have to be able to reprogram in orbit and maybe enhance capabilities desired in the future.”
LIVING BETTER, LIVING LONGER
Greater mission longevity is one of the key improvements GPS III delivers over those currently in service. Space Vehicles 1–10 have a planned mission life of 15 years, 25 percent longer than their predecessors. That begs the question, “How long should a satellite live in space, with technology innovation occurring almost annually?”
Advanced payload technology provides a partial answer. Lockheed Martin and Harris point to new payload capabilities with built-in flexibility to adapt satellites in orbit to technology advances, as well as changes in missions. According to Harris, the fully digital navigation payload will provide the ability to change and upgrade the satellites incrementally over mission life.
In late 2017, Lockheed announced a partnership with NEC Corporation to introduce artificial intelligence for computer learning in orbit. The company touted significant advances in processors and a move toward next-generation antennas, arrays and transmitters to drive more satellite flexibility, capability and resilience.
FROM THE GROUND UP
GPS IIIF’s M-Code can be broadcast from a high-gain directional antenna in a concentrated, high-powered spot beam, in addition to a wide-angle, full-Earth antenna. (Artist rendering: Lockheed Martin)
GPS III’s military upgrades require new ground control stations, a replacement effort called OCX that has suffered repeated delays and cost increases, due to the complexity of the programming and requirements modifications. The new jamming-resistant military signal will not be available until the new, highly complex ground control system is available, and that is not expected until 2022 or 2023. Delay and cost considerations were driven in part by full implementation of all Department of Defense 8500.2 “Defense in Depth” information assurance standards without waivers, giving it the highest level of cybersecurity protections of any DoD space system.
Deliverables for GPS OCX are divided into three blocks. Block 0 delivery took place in fall 2017, enabling it to support the December launch. Block 1 delivery will take place in 2021, providing full operational capability to control both legacy and modernized satellites and signals. Block 2, delivered concurrently with Block 1, adds operational control of L1C and modernized M-code.
In 2018, wrote Col. Whitney of the GPS Directorate, “We have actively utilized the [Block 0] system in a variety of exercises, training events, compatibility tests and launch readiness events. We also completed a comprehensive security review of the system to demonstrate our readiness to start operations. The system is ready to go. We continue to work the development of the OCX Block 1 system and are wrapping up the initial coding of the system early in 2019, leading into our integration and test campaign.”
Given delays in OCX, “the Directorate is actively working two major upgrades to bridge the gap,” Whitney continued. “The first is GPS III Contingency Operations (COps) modification which will allow the 2nd Space Operations Squadron (2SOPS) to command and control the GPS III family of vehicles in a mission state matching today’s legacy signals for all users world-wide. The second modification is M-code early use (MCUE), which enables 2 SOPS to operationalize the Modernized GPS military (M-code) navigation signals for the warfighter.”
Before December’s launch, OCX underwent rigorous cybersecurity vulnerability assessments that tested the system’s ability to defend against both internal and external cyber threats. GPS OCX prevented the broadcast of corrupt navigation and timing data in all tests, bolstering the program’s readiness for GPS III.
“We’ve built a layered defense and implemented all information assurance requirements for the program into this system,” said Dave Wajsgras, president of Raytheon Intelligence, Information and Services. “The cyber threat will always change, so we’ve built OCX to evolve and to make sure it’s always operating at this level of protection.”
The new Harris navigation payload offers a smooth transition to use of OCX. The payload for the first 10 GPS III satellites has been verified for OCX compatibility so the same OCX commands will seamlessly port to the Harris fully digital design, minimizing integration risks and associated costs.
According the the GAO, “Full M-code capability —which includes both the ability to broadcast a signal via satellites and a ground system and user equipment to receive the signal — will take at least a decade once the services are able to deploy military GPS user equipment (MGUE) receivers in sufficient numbers.” The April 2019 issue of GPS World will review M-code implementation across U.S. DoD platforms.
THE FUTURE’S NOT OVER YET
In spring 2018, Lockheed Martin submitted a proposal for the GPS III Follow On (GPS IIIF) program, which will add enhanced capabilities to the satellites. New hardware — a high-gain directional antenna — aims signals in a spot beam at a limited area, but blasts the signal at high power for strategic use by the military.
Inter-Satellite Links. Block IIIF satellites will carry laser retro-reflectors to enable orbit tracking independently of the satellites’ radio signals, which in turn will allow satellite clock errors to be disentangled from ephemeris errors. A standard feature of GLONASS, this is included in the Galileo positioning system, and was flown as an experiment on two older GPS satellites, 35 and 36.
In September 2018, the Air Force selected Lockheed Martin to build up to 22 additional satellites under the GPS IIIF program.
The U.S. Air Force’s second new GPS III satellite, bringing higher-power, more accurate and harder-to-jam signals to the GPS constellation, has arrived in Florida for launch.
On March 18, Lockheed Martin shipped the Air Force’s second GPS III space vehicle (GPS III SV02) to Cape Canaveral for an expected July launch.
Designed and built at Lockheed Martin’s GPS III Processing Facility near Denver, the satellite traveled from Buckley Air Force Base, Colorado, to the Cape on a massive Air Force C-17 aircraft. The Air Force nicknamed the GPS III SV02 “Magellan” after Portuguese explorer Ferdinand Magellan.
GPS III is the most powerful and resilient GPS satellite ever put on orbit. Developed with an entirely new design, for U.S. and allied forces, it will have three times greater accuracy and up to eight times improved anti-jamming capabilities over the previous GPS II satellite design block, which makes up today’s GPS constellation.
GPS III also will be the first GPS satellite to broadcast the new L1C civil signal. Shared by other international global navigation satellite systems, like Galileo, the L1C signal will improve future connectivity worldwide for commercial and civilian users.
The Air Force began modernizing the GPS constellation with new technology and capabilities with the December 23, 2018 launch of its first GPS III satellite. GPS III SV01 is now receiving and responding to commands from Lockheed Martin’s Launch and Checkout Center at the company’s Denver facility.
Lockheed Martin shipped the U.S. Air Force’s first GPS III to Cape Canaveral, Florida ahead of its expected July launch. (Photo: Lockheed Martin}
“After orbit raising and antenna deployments, we switched on GPS III SV01’s powerful signal-generating navigation payload and on Jan. 8 began broadcasting signals,” Johnathon Caldwell, Lockheed Martin’s Vice President for Navigation Systems. “Our on orbit testing continues, but the navigation payload’s capabilities have exceeded expectations and the satellite is operating completely healthy.”
GPS III SV02 is the second of ten new GPS III satellites under contract and in full production at Lockheed Martin. GPS III SV03-08 are now in various stages of assembly and test. The Air Force declared the second GPS III “Available for Launch” in August and, in November, called GPS III SV02 up for its 2019 launch.
In September 2018, the Air Force selected Lockheed Martin for the GPS III Follow On (GPS IIIF) program, an estimated $7.2 billion opportunity to build up to 22 additional GPS IIIF satellites with additional capabilities.
GPS IIIF builds off Lockheed Martin’s existing modular GPS III, which was designed to evolve with new technology and changing mission needs. On September 26, the Air Force awarded Lockheed Martin a $1.4 billion contract for support to start up the program and to contract the 11th and 12th GPS III satellite.
Once declared operational, GPS III SV01 and SV02 are expected to take their place in today’s 31 satellite strong GPS constellation, which provides positioning, navigation and timing services to more than four billion civil, commercial and military users.
Less than three weeks after its launch, the first GPS III satellite, SVN74, started transmitting navigation signals. SVN74 uses the pseudorandom noise (PRN) code number G04 previously used by the almost 25-year-old Block IIA satellite SVN36. The L1 C/A, L1 P(Y), and L2 P(Y) signals of SVN74 have been tracked since Jan. 9 at 00:01 UTC. Activation of the L2C and L5 signals followed on the same day at 19:43 UTC. Transmission of the legacy navigation message (LNAV) started Jan. 9, but the satellite is still marked unhealthy for ongoing on-orbit check out and testing.
Also, SVN74 is the first GPS satellite to transmit a new civil signal on the L1 frequency (1575.42 MHz), namely L1C, which was initially activated on the same day as the other SVN74 signals. Incidentally, the L1C signal was already being transmitted by the four satellites of the Japanese Quasi-Zenith Satellite System (QZSS).
Compared to the L1 C/A PRN codes, the L1C codes are 10 times longer (10,230 chips), reducing interference when multiple satellites are tracked by a receiver on the same frequency. Like L2C and L5, the L1C signal consists of a dataless pilot component and the data component with navigation data. Dataless signals enable more robust tracking under difficult conditions. For the L1C signal, 75 percent of its power is put into the pilot component.
The theoretical spectra of the four signals transmitted on L1 by SVN74, namely the civil C/A-code and L1C, as well as the military P(Y)-code and M-code, are shown in FIGURE 1 along with the the total (summed) spectrum.
Figure 1. Theoretical spectra of the four signals transmitted by a GPS III satellite in the L1 frequency band. (Image: Authors)
BOC. To achieve compatibility with the L1 C/A-code signal at the same center frequency, a binary offset carrier (BOC) modulation is used for spectral separation of L1C from L1 C/A. A BOC(n,m) signal is characterized by the fundamental frequency of the square wave subcarrier expressed in multiples n of the basic frequency of 1.023 MHz and the chipping rate expressed in multiples m of 1.023 megachips per second. A BOC(1,1) modulation is used for the L1C data component. For the pilot component, a time-multiplexed binary offset carrier (TMBOC) is used. The spreading waveform, with a length of 33 symbols, consists of four BOC(6,1) and 29 BOC(1,1) symbols as illustrated in FIGURE 2 resulting in a TMBOC(6,1,4/33) signal. The additional BOC(6,1) component allows for improved multipath mitigation.
Figure 2. Spreading symbols for the L1C pilot component: time-multiplexed BOC consisting of BOC(6,1) for the 1st, 5th, 7th and 30th symbols and BOC(1,1) for the other symbols. (Image: Authors)
Similar to GPS L1C, the European Galileo and the Chinese BeiDou-3 systems employ multiplexed BOC signals with BOC(1,1) and BOC(6,1) components in the L1 frequency band. A composite BOC (CBOC) modulation has been adopted for the Galileo E1 open service signal, which uses a weighted sum of the BOC(1,1) and BOC(6,1) components in both the data and the pilot channels. For the BeiDou B1C signal, BOC(1,1) is used for the data channel, while a quadrature multiplexed BOC modulation, QMBOC(6,1,4/33), with BOC(1,1) and BOC(6,1) subcarriers in phase quadrature, is used for the pilot channel.
Interoperability. The new civil L1 signals of GPS, Galileo and BeiDou show a high level of commonality and are specifically designed for full interoperability. This means that receivers can easily track signals of all three constellations and use the measurements to compute a combined multi-GNSS position solution. Aside from the similar signal modulations, the interoperability is further supported by the transmission of inter-system timing biases (such as the GPS-Galileo Time Offset) in the navigation messages.
The binary phase shift keying (BPSK) modulation of the C/A-code with a 1.023-MHz chipping rate introduces a main lobe at the center frequency of 1575.42 MHz and numerous side lobes with decreasing amplitude. The 10.23-MHz BPSK signal of the P(Y)-code is visible in Figure 1 as a broad peak at the center frequency and first side lobes at about 1560 and 1590 MHz. The M-code is characterized by its main lobes ±10.23 MHz from the center frequency due to its BOC(10,5) modulation. Finally, the L1C signal can be recognized as two narrow peaks separated by ±1.023 MHz from the L1 center frequency related to the BOC(1,1) modulation and two peaks at ±6.138 MHz related to the BOC(6,1) modulation. Side lobes of the BOC(1,1) signal are visible next to the main lobes at integer multiples of 2 × 1.023 MHz.
Observations. The German Aerospace Center (DLR) operates a 30-meter dish antenna at its ground station in Weilheim, near Munich, Germany. FIGURE 3 shows the L1 spectrum of SVN74 measured on January 15, 2019. One can clearly see the L1C BOC(1,1) main lobes at 1574 and 1576 MHz as well as the BOC(6,1) main lobes at 1569 and 1581 MHz. Selected side lobes are also indicated.
Figure 3. SVN74 L1 spectral flux density measured with the Weilheim 30-meter antenna on January 15, 2019, at 08:04 UTC. Selected features of the L1C signal are indicated by arrows. (Image: Authors)
Initially, none of the International GNSS Service network receivers could track the L1C live signal of SVN74, but dedicated firmware versions supporting L1C tracking were soon made available by selected manufacturers. FIGURE 4 shows the multipath linear combination for the L1 C/A-code and the L1C signal tracked with a Javad TRE-G3TH receiver. Reduced measurement noise (multipath plus receiver measurement noise) of the L1C signal can be seen over all elevation angles ranging from about 3 to 83 degrees. (Tracking of the pass began at 4.3 degrees and ended at 3.0 degrees.)
Figure 4, Multipath linear combination (L1 pseudorange and L1 and L2 carrier phase) of the SVN74 L1 C/A-code (top) and L1C signal (bottom) from 1-Hz data of February 3, 2019, tracked with a Javad TRE-G3TH receiver at the Geodetic Observatory Wettzell. (Image: Authors)
The overall root-mean-square noise of the SVN74 pass shown in Figure 4 is 32 centimeters for the L1 C/A-code signal and 24 centimeters for L1C, that is, a reduction of 25 percent for L1C. Compared to the BPSK modulation of the legacy C/A-code signal, the increased steepness of the TMBOC correlation function offers lower measurement noise for the L1C tracking. In addition, the sensitivity to multipath is reduced.
CNAV-2. Together with L1C, the second version of the civil navigation message, namely CNAV-2, is being transmitted. CNAV-2 is composed of three subframes: subframe 1 contains information about the current epoch. Subframe 2 comprises clock and ephemeris data including inter-signal corrections (ISCs). ISCs provide clock corrections for single-frequency users and dual-frequency users utilizing signals other than L1 P(Y) and L2 P(Y). Whereas the essential broadcast ephemeris data in subframe 2 repeat continuously over the validity period of typically two hours, subframe 3 contains pages with alternating content as listed in TABLE 1 (page 41).
Table 1 Currently defined pages of the CNAV-2 subframe 3.
Despite a different message layout, most CNAV-2 parameters and their values match those transmitted in the CNAV message of the L2C and L5 signals. Additional parameters comprise the ISCs for the L1C signal. Compared to the LNAV legacy navigation message, CNAV and CNAV-2 utilize an extended set of ephemeris parameters that allow for a smoother orbit representation compared to LNAV. Multi-GNSS applications benefit from the GPS/GNSS time offset (GGTO) parameters included in page 2. In the same page, Earth orientation parameters are provided that are relevant for users of an inertial frame, such as for spaceborne navigation. The CNAV-2 repeat cycle of 18 seconds allows for a faster access to broadcast ephemerides included in subframe 2 compared to LNAV. Compared to CNAV, CNAV-2 furthermore provides a more sophisticated error detection and correction scheme.
As of the beginning of February 2019, only pages 1, 2 and 4 of CNAV-2 subframe 3 are being used. Within a cycle of 144 seconds, page 1, page 2 and six sets of page 4 midi almanac data (each for one individual satellite) are transmitted. The full almanac for 32 satellites is thus transferred in an average of about 13 minutes. The content in these subframes corresponds to that in L2 and L5 CNAV messages. Updates of CNAV-2 are performed in two-hour intervals starting at 01:30. This is the same update scheme as for CNAV but different from LNAV where the two-hour intervals start at 00:00.
Note that some time will pass before enough GPS III satellites are transmitting so that users can fully enjoy the benefits of the new L1C signal.
MANUFACTURERS
Spectral measurements at the Weilheim 30-meter antenna were made with a Rohde & Schwarz FSQ26 vector signal analyzer. Receiver measurements have been collected with a JAVAD GNSS TRE-G3TH receiver running an L1C-capable firmware version.
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.
