A mix of old and new satellites make up the current GPS constellation, with GPS III satellite launches and orbital insertions taking place according to plan, though the journey to deliver the system’s other segments has been less smooth. This article looks at some of the latest efforts, associated activities undertaken and scrutiny to deliver an improved, next-generation GPS system.
GPS has been the mainstay of civil and military positioning, navigation and timing (PNT) purposes since the mid-90s. The satellite-based radio navigation system delivers benchmark PNT services to US and allied militaries, as well as civilian users. Over four billion people from civil sectors such as finance, transportation, recreation and agriculture – together with the military sector – rely on PNT radio signals in their everyday activities and operations.
The decision to procure the future generation of GPS satellites, known as GPS III, was made in September 2007. In August 2021, day-to-day responsibility for modernising and sustaining GPS transitioned from the US Air Force to Space Force’s Space Systems Command (SSC), with the US Government committed to maintaining the availability of ‘at least’ 24 operational GPS satellites, ‘at least’ 95% of the time. This service-level commitment has been ensured by the US typically keeping 31 GPS satellites operational for well over a decade now.
The current modernisation programme covers the GPS system’s three segments: ground control segment, space segment, and user equipment, with the effective interoperation of these segments designed to provide the very latest and improved levels of PNT service, as well the military-specific, Military-code (M-code) signal. M-code is a stronger, encrypted, GPS signal designed to meet military PNT information needs and will help military users overcome attempts to jam the GPS signal by using a more powerful signal with split-spectrum waveform. It will also protect against GPS spoofing, by encrypting the signal. SSC manages the acquisition of the space segment portion of this effort through the GPS III and GPS III Follow-On (GPS IIIF) programmes, which manage the replacement of existing satellites as they near the end of their intended operational life.
The latest activities to upgrade the system to GPS III have seen six successful GPS III satellite insertions into orbit, (although the first launch in 2018 came after a four-year delay), with the sixth launched in January 2023 and the tenth delivered in February. As a result, the GPS space segment currently consists of a constellation that includes older satellites, active before the current modernisation, and now, the first six GPS III satellites.
Ground control and user-equipment segments, however, have had their own delays and an early June report by the US Government Accountability Office (USGAO) has called several aspects of the programme upgrade into question, with recommendations made, more of which below.
Space Segment Developments
Delivery and launches of the space segment’s GPS III satellites have gone well since the initial delays prior to first launch and the GPS satellite constellation today comprises four generations of satellites with varying capabilities and design lives. Interestingly, the first satellite capable of transmitting the M-code signal entered orbit as far back as 2005 and as of now, 25 of the constellation’s current 31 active satellites are M-code capable. However, to ensure that the constellation maintains and expands this signal capability it looks like additional M-code-capable satellites will need to join the constellation.
The first GPS III satellite, built by Lockheed Martin, was launched in 2018 and, in mid-January 2023, the sixth GPS III satellite, Space Vehicle 06 (GPS III SV06), took off from Cape Canaveral Space Force Station in Florida aboard a SpaceX Falcon 9 rocket. As with the previous five GPS III satellite launches, the SV was sent into space under the control of US Space Force personnel and Lockheed Martin engineers at the company’s Denver Launch & Checkout Operations Centre, and was eventually placed in operational orbit approximately 20,197 km above the Earth. Around 90 minutes after launch, control of the satellite is assumed by the ground control team, with the satellite’s onboard liquid apogee engine propelling the satellite towards its operational orbit over the following days. Once it arrives in orbit, engineers send the SV commands to deploy its solar arrays and antennas in preparation for handover to Space Operations Command. GPS III SV06 was the 25th M-code satellite introduced to the constellation.
For its part, Lockheed Martin has now completed production on its original contract to deliver 10 GPS III satellites, SVs 1-10. The Space Force confirmed at the end of 2022 that SV10 had joined SVs 7-9 in being completed and were ready to be launched at a later date; all four satellites remain in storage at the maker’s facility in Waterton, Colorado, until the Space Force calls them forward for their respective launches. It’s worth noting that the GPS III team is led by the Space Production Corps Medium Earth Orbit Division at the US Space Force’s Space and Missile Systems Centre, Los Angeles Air Force Base (AFB), while GPS Operational Control Segment support is managed by the Enterprise Corps, GPS Sustainment Division at Peterson AFB. The 2nd Space Operations Squadron, at Schriever AFB, manages and operates the GPS constellation for both civil and military users.
GPS III satellites and associated systems have been developed with a modular design, factoring changing operational requirements and emerging threats into the mix, thereby allowing for new technology and capabilities to be added in the future. As such, they’ve been able to provide significant capability advancements over earlier-designed GPS satellites in orbit, including three times better accuracy, up to eight times improved anti-jamming capabilities, as well as improved L1C civil signal, which is compatible with international global navigation satellite systems (GNSSs), such as Europe’s Galileo, to improve civilian user connectivity.
So, with GPS III SV1 launched in 2018 and SV6 in January this year, there are now six operational GPS III satellites in orbit, each with a 15-year design lifespan and currently transmitting Block IIF signals with enhanced signal reliability, accuracy and integrity. The improved L1C frequency, the fourth civilian GPS signal, is designed to enable interoperability between GPS and international satellite navigation systems. L1C, which has now been broadcasting from the six GPS III satellites since 3 July 2023, refers to the radio frequency used (1575 MHz, or L1). Two military signals also exist at L1 (P/Y-Code and M-Code), as well as the legacy civil C/A-Code signal. L1C allows use of the multiplexed binary offset carrier modulation (MBOC) waveform, which is interoperable between GPS’ L1C and with Galileo’s E1 OC frequencies. This is intended to improve mobile GPS/Galileo reception in cities and other complex environments.
It is also worth noting that selective availability (SA), which had been a trait of the early years of GPS impacting civilian users with an intentional degradation of public GPS signals for US national security reasons, is no longer a feature of GPS service, the decision having been taken in 2007 not to include it in the GPS III upgrade. As of 3 July 2023, there were a total of 31 operational satellites in the GPS constellation, not including seven decommissioned, or in-orbit spares.
(Credit: Lockheed Martin)
Lockheed Martin has begun building the first of the GPS III Follow On (GPS IIIF) satellites, which are set to feature new capabilities, such as a laser retroreflector array to enhance accuracy, a new search and rescue (SAR) payload, and a digital navigation payload. The first is due to be launched in 2027, and Lockheed Martin has been contracted to build 10 satellites. To clarify, numbering of the SVs used by the GPS IIIF series starts at SV11, with SV11-20 contracted so far.
Ground Control Segment
The ground control segment of GPS is made by Raytheon and consists of the ground-based stations that are responsible for tracking, monitoring, and updating the satellites. The enhanced ground control segment is referred to as GPS Next-Generation Operational Control System or GPS OCX, which will provide improved accuracy of the current system and will be able to fly more than twice as many satellites, an increase intended to improve coverage in hard-to-reach areas such as urban and mountainous terrain. According to the manufacturer, OCX also offers improved accuracy, availability, and resistance to jamming compared to the previous ground control segment.
As far as GPS OCX system deliverables are concerned, Raytheon has stated that these are divided into three blocks: Block 0, Block 1 and Block 2, with the first of these having taken place in late 2017, enabling the GPS OCX Launch and Checkout System (LCS) to support the first GPS III satellite launch in December 2018. Since then, according to Raytheon, the LCS has been used to launch three GPS III satellites.
However, there have been continuing OCX schedule delays, (reported to be seven years so far), and cost overruns, (the current price tag of USD 7 Bn is reported to be some 73% more than the US Government initially projected), to the delivery of 17 ground stations. The USAF informed Congress in mid-2016 that the OCX programme’s projected costs had risen above USD 4.25 Bn, exceeding the initial estimates of USD 3.4 Bn by 25%. One of the factors said to have led to these overruns was the complexity of the OCX’s cybersecurity requirements; nevertheless, in October 2016, the DOD formally certified the programme, which was necessary for it to continue.
Deliveries of Blocks 1 and 2 of the OCX, expected in January 2023, have been delayed as a result of technical problems discovered during testing. According to a report published in June 2023, this will push the date for initial operating capability back to sometime in the spring of 2024. OCX has been designed with the intention of operating the entire GPS fleet, but that will be when it is fully functioning with all Block increments. While Block 0 can and has handled the GPS III launches, it cannot operate the satellites in orbit and only when Blocks 2 and 3 are up and running will that be possible. Nevertheless, the Space Force hopes to complete operational acceptance for all elements of the OCX segment in 2027, though only time will tell if that timeframe is met.
Scrutiny of GPS III
Considering the US DOD has been working to upgrade GPS with M-code for many years and as far back as 2005 launched the first M-code-capable GPS satellite, it’s hardly surprising delays and cost increases to different parts of the programme have drawn attention, frustration and questions from government quarters. As a result, the US Congress tasked the US Government Accountability Office (USGAO), to assess the cost, schedule and performance of the various current GPS acquisition programmes, the result of which was an oversight report published in June 2023, titled: ‘GPS Modernization – Space Force Should Reassess Requirements for Satellites and Handheld Devices’.
Of the ground segment, the GAO said the Space Force had further delayed delivery of the fully operational ground control segment in 2022 due to “development challenges”, with the delay pushing delivery back until December 2023, at the very minimum. Indeed, the report said that Space Force officials had not, at that stage, even finalised a new schedule and acknowledged that remaining risks could lead to additional delays.
Of the space segment, on the other hand, the GAO report was more upbeat, saying that the Space Force had met its approved requirement to have “24 M-code-capable satellites on orbit”, but went on to say that at least three further satellites would be needed for the upgraded GPS system “to meet certain user requirements for accuracy”, though it acknowledged “[b]uilding and maintaining this larger constellation presents a challenge”. In its analysis, the GAO suggested it is unlikely for 27 satellites to be consistently available over the next decade and that unless the USAF assesses its operational need for satellites to establish a firm, 27-satellite-constellation requirement, other DOD programmes might well take priority. The implications of such a scenario is that GPS equipment used by friendly forces could perform below their required capability levels.
Concerning that user equipment, Military GPS User Equipment Increment 1 (MGUE Inc 1) development has progressed to the point where military departments are ready to commence activities in support of testing and fielding it on key weapon systems. However, it added that delays and unexpected challenges could affect the fielding the capability on some platforms. Indeed, the GAO report stated that while, “Space Force seeks to expand the use of M-code technology by developing a second increment consisting of an improved M-code chip and card, as well as a handheld receiver… it lacks a major committed customer for the handheld receiver.”
The GAO said that the US Army, the largest potential user of such a device, has its own plans for handheld receivers, and the USMC is, apparently, also still considering its options. This lack of a solid business case for its proposed handheld device, risks significant resources and costs being expended by Space Force with little or no benefit to military end users, according to the GAO report.
As for the military’s response to the GAO’s findings and recommendations, in an appendix to the organisation’s report, head of the Air Force Space Acquisition and Integration Office, Frank Calvelli, accepted, in their entirety, the two GAO findings that stated, firstly, that the service should consider officially adding three more M-Code-capable satellites to its current operational requirement something also approved by the Space Force. Secondly, he accepted those findings recommending the Space Force either re-work its requirements, or drop plans altogether to develop the new, smaller M-code chip card and specialised GPS receiver for handheld radios; the lack of a sound business case and because the other services, as mentioned, have their own plans were understood as the reasons for this recommendation for an alternative approach.
NTS-3 Testing to Support GPS
The Space Force announced in June 2023 that the Benefield Anechoic Facility (BAF) at Edwards AFB was in the process of testing Navigation Technology Satellite-3 (NTS-3), its first experimental navigation satellite system in nearly 50 years. Developed by L3Harris Technologies, NTS-3 was originally slated for launch in late-2023, but this has now been delayed until late-2024.
NTS-3 will demonstrate technologies and techniques to augment the GPS constellation and help maintain access to GPS in contested environments. Testing at the BAF is also an important step in ensuring that the AFRL team is ready to successfully conduct in-orbit experiments. The USAF, US Space Force, AFRL, and NASA’s Jet Propulsion Laboratory have all had critical roles to play in the testing of the NTS-3 satellite. After the testing is complete, the NTS-3 will operate for one year in a near-geosynchronous orbit and will broadcast navigation signals from its phased array antenna.
Tim Guest
