Assured PNT beyond GPS: M-code, LEO, eLoran, inertial, and quantum options

For most of the last thirty years, the military answer to "where am I, which way am I going, and what time is it" has been the same: GPS. It is accurate, free at the point of use, global, and baked into almost every weapon, vehicle, radio, and network the Department of Defense owns. That ubiquity is also the problem. GPS signals arrive at the earth's surface at very low power from satellites in medium earth orbit, and any force that relies on a single low-power signal for position and timing has handed an adversary an obvious target for electromagnetic attack.

The department has known this for a long time. The response has two halves. The first is to harden GPS itself through new satellites, a new ground control segment, and military code (M-code) user equipment. The second is to stop treating GPS as the only source of truth and build user equipment that fuses GPS with inertial sensors, clocks, terrestrial and space-based signals, and onboard sensing of the environment. Together these are usually grouped under the label assured positioning, navigation, and timing, or A-PNT.

This article walks through both halves using public sources: what A-PNT means in DoD usage, where GPS modernization actually stands according to GAO and DOT&E, what the Army and other services have fielded, which alternative sources are real and which are still in the lab, why integration is the hard part, and what program offices should be watching over the next few years.

What "assured PNT" means in DoD usage

Assured PNT is not a single system. It is a property of a system or a force. A commonly cited DoD definition, quoted in a 2021 Defense Systems Information Analysis Center (DSIAC) technical inquiry report on A-PNT, describes it as the ability to provide operational forces continuous access to position, velocity, attitude, and time information with confirmed integrity and sufficient accuracy to perform the mission under the complete range of threat conditions. The threat conditions in that framing explicitly include GPS degradation, denial, and deception.

Several words in that definition carry most of the weight:

Continuous. A receiver that is accurate until it is jammed and then fails silently is not assured. The goal is graceful degradation, not a cliff.
Confirmed integrity. Knowing your position is wrong is often more valuable than a slightly better position. Spoofing is dangerous precisely because it produces confident, wrong answers.
Sufficient accuracy. Not GPS-level accuracy everywhere. A howitzer, a convoy, a fires network timing source, and a strike weapon all need different things. GAO made this point directly in 2021 when it noted that default assumptions about GPS-level precision may exceed actual mission needs.
Time, not just position. Timing is the quiet half of PNT. Frequency-hopping radios, networks, cryptographic systems, and many sensors depend on precise time, and timing users often lose GPS without anyone noticing until something downstream breaks.

In practice, the services describe A-PNT as a layered approach. GPS, preferably M-code, remains the primary source when it is available. Around it sit protected antennas and receivers that can detect and reject interference, inertial measurement units and odometry that carry the solution through outages, stable clocks that hold time, and alternative sources that provide independent absolute fixes. A fusion engine weighs all of them, flags when one source disagrees with the others, and tells the operator how much to trust the answer.

GAO's 2021 technology assessment on defense navigation capabilities, which surveyed the department's work on complements to GPS, sorted alternatives into two broad families. Relative PNT sources, such as inertial sensors and clocks, measure change from a known starting point and drift over time. Absolute PNT sources, such as celestial, magnetic, very low frequency radio, and low earth orbit (LEO) satellite signals, provide independent fixes that can bound that drift. GAO concluded that no single alternative can replace GPS across all military applications, which is the basic logic behind layering.

GPS modernization: where it actually stands

Before looking at alternatives, it helps to be clear about the status of GPS modernization, because many A-PNT plans assume M-code is available and fielded. The public record shows steady but slow progress across all three segments of the system.

Space segment: GPS III and GPS IIIF

M-code is not new on orbit. GAO reported in September 2024 (GAO-24-106841) that the first M-code capable satellite launched in 2005, and that as of May 2024, 24 of 31 satellites in the operational constellation were M-code capable. The Space Force's stated goal, per GAO, is to keep 24 M-code capable satellites in continuous operation through the 2030s, and GAO warned that the service risks missing that goal because of delays in follow-on satellites.

The GPS III satellites built by Lockheed Martin have been launching steadily. GPS III SV-08 launched on May 30, 2025, on an accelerated schedule, leaving two GPS III satellites remaining at that time according to GPS World reporting. The follow-on GPS IIIF satellites add capabilities, including a regional military protection signal intended to deliver higher-power M-code into a theater of operations. GAO reported in 2024 that technical and manufacturing challenges, including problems with traveling wave tube amplifiers, had pushed the planned first GPS IIIF launch later. DOT&E's FY2025 annual report lists the first GPS IIIF launch for FY2027 with operational acceptance in FY2028.

Control segment: OCX

The Next Generation Operational Control System (OCX), built by RTX, is the ground system needed to command the modernized constellation and enable full M-code operations. It is one of the department's most troubled software programs. GAO's 2024 report described continuing deficiencies found in qualification testing and an operational acceptance date that had moved to December 2025. Breaking Defense reported in July 2025 that RTX delivered OCX Blocks 1 and 2 to the Space Force on July 1, 2025, that total projected program cost was about $10 billion, and that the program was more than $3 billion over budget.

Delivery is not the same as operational use. DOT&E's FY2025 report states that the Space Force accepted OCX from the contractor in July 2025, that operational testing began with a cooperative vulnerability and penetration assessment in September 2025, and that a constellation transfer from the legacy control system is planned in FY2026 with operational acceptance to follow. DOT&E also stated that there were insufficient data at that time to assess OCX's operational effectiveness and suitability, and that continued OCX delays put U.S. warfighters and allies at risk because full M-code capability has not yet been operationally deployed.

User segment: MGUE and service receivers

The user segment is where most warfighters feel the delay. The Military GPS User Equipment (MGUE) program develops the M-code receiver cards and application specific integrated circuits (ASICs) that the services then integrate into their own platforms.

MGUE Increment 1 covers ground and aviation/maritime receiver cards. GAO reported in 2024 that ground card development was complete after years of delay, while the aviation/maritime card still faced schedule risk and a recently discovered deficiency. DOT&E's FY2025 report notes that U.S. Army Special Operations Command conducted integrated developmental and operational testing of MGUE Increment 1 as configured on the Gray Eagle unmanned aircraft in September 2025, and that more testing remains on maritime platforms and on aviation platforms carrying GPS-guided weapons, such as the B-2, before a full operational assessment.
MGUE Increment 2 aims at smaller, lower-power chips for handheld devices and precision munitions. GAO reported in 2024 that one of three contractors had withdrawn in October 2023 and that the effort risked exceeding the five-year middle tier of acquisition window.
Receiver card supply. GAO flagged a potential gap in the availability of GPS receiver cards as legacy cards go out of production before M-code replacements are ready. GAO noted that DoD's estimate of how long a bulk buy of Increment 1 chips would last had been revised, and that higher demand driven by global conflicts could deplete supplies sooner than planned.

Service-level integration varies. In the same 2024 report, GAO found the Army on track for M-code fielding in its ground systems, the Navy relying on Air Force receivers for many platforms and planning maritime receiver testing in 2025, and the Air Force's receiver efforts significantly delayed, including the end of one embedded GPS/INS contract in March 2024 after the vendor could not provide an executable schedule.

The practical takeaway is that M-code is real, it is on orbit, and receivers are reaching units, but the full end-to-end capability that depends on OCX and broad receiver fielding has been arriving years later than planned. A-PNT programs that assume ubiquitous M-code by a specific date should carry that as a risk.

Resilient GPS

The Space Force has also pursued a smaller, cheaper augmentation constellation called Resilient GPS (R-GPS). As publicly reported in early 2025, the effort awarded initial design contracts to four companies in 2024, later narrowed to three, with a goal of launching a first set of satellites around 2028 and a projected cost of roughly $1 billion over five years. The House Appropriations defense subcommittee rejected a $77 million FY2025 funding realignment for the effort in June 2024, writing that it was not clear how the additional satellites increase resilience against the primary jamming threat to GPS, and pointing to the M-code user equipment shortfall. The program's future depends on later budget decisions that are still playing out, so it is worth tracking but not counting on.

Army A-PNT: MAPS, DAPS, and the layered approach

The Army has gone further than any other service in fielding dedicated A-PNT systems as programs of record. The work sits in Project Manager Positioning, Navigation, and Timing (PM PNT) under Program Executive Office Intelligence, Electronic Warfare, and Sensors, with strategy and roadmaps coordinated through what is now the All-Domain Sensing Cross-Functional Team.

Mounted Assured PNT System (MAPS)

MAPS Gen I was first fielded in 2019 as an interim capability to protect vehicle GPS from jamming. MAPS Gen II is the long-term system. Public Army descriptions list M-code, sensor fusion with alternative sources, and an improved exterior antenna with anti-jam and anti-spoofing capabilities, packaged so that a single system can replace both MAPS Gen I and several legacy Defense Advanced GPS Receivers on a vehicle. Inside GNSS reported in 2025 that the system uses a seven-element anti-jam antenna and is intended for vehicles, watercraft, and munition systems.

The Army approved full-rate production of MAPS Gen II in early 2025, under a Collins Aerospace contract first awarded in September 2022 and reported at $583 million over five years. An Army.mil article from September 2025 described fielding underway, including to the 2nd Stryker Brigade Combat Team, 2nd Infantry Division, and quoted the program office describing the system's ability to detect and reject GPS interference and notify the soldier that it is operating in an electromagnetic warfare environment.

Dismounted Assured PNT System (DAPS)

DAPS Gen II is the handheld equivalent: an M-code receiver that fuses multiple PNT sources and works standalone, with a wrist-worn display, or with the Nett Warrior end user device. DOT&E's FY2024 report describes initial operational testing at Fort Huachuca in November 2023, full-rate production approval in August 2024, and a target of initial operational capability in March 2025. DOT&E assessed DAPS Gen II as operationally effective, while noting decreased position and notification accuracy under very challenging threat environments, and found it operationally suitable with 99 percent operational availability during testing.

Scale and the layered model

An Army.mil article from May 2025 reported that in FY2024 the Army fielded about 27,000 M-code capable receivers, more than 2,500 ground A-PNT systems, roughly 7,000 M-code precision guidance kits for artillery, and 46 M-code aviation navigation systems on Black Hawk helicopters. That is significant, but still a fraction of the legacy receiver base.

Army program officials have publicly described the approach as layering: inertial measurement units, wheel speed odometry, chip-scale atomic clocks, multi-constellation reception, M-code, and hardened alternative navigation (ALTNAV) signals, all combined in a fusion engine. Inside GNSS also reported in 2025 on a newer Army effort called NorthStar, described as a modular, open systems approach to lower-cost mounted PNT that can be upgraded as threats change.

Other services

The other services have taken different routes, often embedding A-PNT functions into platform navigation systems rather than standing up separate A-PNT programs.

Air Force. Most aircraft rely on embedded GPS/inertial navigation systems (EGIs). The modernization path runs through M-code EGIs and related receivers, which GAO reported in 2024 were significantly delayed. The Air Force has also funded alternative navigation experiments, including the magnetic anomaly navigation flights described below.
Navy. Ships use the GPS-based Navigation and Timing System (GPNTS) as the M-code path for surface combatants, with GAO reporting operational testing planned for 2025. Submarines and ships have long relied on high-grade inertial navigation and precise clocks, which gives the Navy a different starting point than ground forces.
Marine Corps. The Joint Light Tactical Vehicle was a lead platform for MGUE Increment 1 integration per GAO. Public detail beyond receiver modernization is limited.
Special operations. USSOCOM has been an early tester of M-code equipment, including the Gray Eagle MGUE testing noted by DOT&E, and often acquires commercial A-PNT gear on faster timelines than the services.

Alternative and complementary sources

The alternatives fall into a few groups: other space-based signals, terrestrial radio signals, self-contained sensors that measure the environment, and better clocks and inertial sensors. Each has a different maturity, a different failure mode, and a different integration burden.

Commercial LEO PNT

Satellites in low earth orbit are roughly twenty to thirty times closer to the earth than GPS satellites, which means their signals can arrive much stronger and their geometry changes quickly, which helps some positioning techniques. Several efforts are relevant:

Iridium PNT (formerly Satelles STL). Satelles built a Satellite Time and Location service on Iridium's 66-satellite LEO constellation. Iridium agreed in March 2024 to acquire the remaining stake in Satelles for about $115 million, and Satelles leadership described the signal as roughly 1,000 times stronger than GPS at the time. The service is already used for timing and location assurance in commercial settings and has been part of DOT's complementary PNT testing.
Purpose-built LEO navigation constellations. Xona Space Systems launched its first production satellite, Pulsar-0, on July 1, 2025, on SpaceX's Transporter-14 rideshare. The company has described plans for a constellation of hundreds of satellites with authenticated, higher-power signals. The company also disclosed that it launched without onboard propulsion to hold schedule, accepting reduced mission life. These systems are early, and military use would depend on accreditation, coverage, and resistance to interference that has not yet been publicly demonstrated at scale.

LEO PNT is not immune to electromagnetic attack. Stronger signals raise the bar for a jammer, but they do not remove the threat, and the receiving equipment still needs to be protected, integrated, and certified. It also creates dependence on commercial operators, which raises questions about service guarantees in conflict and how to write requirements against a service the government does not control.

eLoran and Loran in the United States

Loran-C, a high-power, low-frequency terrestrial navigation system, operated in the United States for decades. The Coast Guard announced the decision to decommission it in January 2010, citing costs and redundancy with GPS. Enhanced Loran (eLoran), a modernized version with better timing and data channels, has been proposed repeatedly since then as a domestic GPS backup, particularly for timing.

As of the most recent public federal positions, the United States does not operate an eLoran service. The Department of Transportation has taken a technology-agnostic position. In a November 2023 letter responding to industry concerns, DOT stated that it believes there is a need for multiple and diverse PNT technologies for critical infrastructure, and that its October 2023 request for information on eLoran did not state a preference for eLoran. That RFI was tied to the Coast Guard's required divestiture of former Loran sites under the FY2023 National Defense Authorization Act, and sought information on federal assets that a commercial eLoran service would need.

For DoD, eLoran is relevant mainly as a homeland and critical infrastructure timing source, and as an example of a terrestrial, high-power signal that is hard to jam over wide areas. It is not a deployable battlefield navigation aid in its traditional form, because transmitters are large, fixed, and would be targets.

Terrestrial and signals-of-opportunity navigation

Beyond Loran, several terrestrial approaches exist:

Pseudolites. Ground-based transmitters that broadcast GPS-like signals over a local area. They can provide a local constellation when space signals are denied, at the cost of being emitters that can be located and targeted.
Commercial terrestrial PNT networks. Companies have built terrestrial beacon networks for urban and indoor positioning and timing. NextNav, for example, petitioned the Federal Communications Commission in April 2024 to reconfigure the Lower 900 MHz band (902 to 928 MHz) to support a terrestrial complement and backup to GPS. These are domestic infrastructure efforts, not expeditionary capabilities.
Signals of opportunity. Receivers can extract position and timing information from signals never intended for navigation, such as cellular, broadcast television and radio, and satellite communications. The advantage is that the signals already exist and are hard to deny everywhere. The disadvantages are that the transmitters are not under friendly control, their locations and clocks must be known or estimated, and an adversary can turn them off or manipulate them in a theater it controls.
Fiber and network timing. For fixed sites, precise time can be distributed over fiber using protocols such as Precision Time Protocol. DOT's 2023 complementary PNT action plan described two fiber timing systems among the 11 technologies it had demonstrated.

DOT's complementary PNT work is the best public picture of how these perform. The 2023 action plan summarized earlier demonstrations of 11 technologies, including two LEO systems, two fiber timing systems, a system combining map matching, inertial sensing, and ultra-wideband, and six terrestrial radio frequency systems. A follow-on rapid phase awarded more than $7 million to nine vendors in June 2024 for field testing. According to DOT's Volpe Center, that testing concluded in June 2025 and results were briefed at an interagency workshop in August 2025.

Celestial navigation

Celestial navigation measures the angles to stars and other bodies and, with an accurate clock and vertical reference, computes position. It is passive, emits nothing, and cannot be jammed in the usual sense. Astro-inertial systems have flown on strategic aircraft and missiles for decades.

The limits are well understood. Daytime star tracking and cloud cover are hard problems for small, low-cost sensors near the ground. Accuracy depends on knowing the local vertical, which is difficult on a moving vehicle. Celestial works best on aircraft at altitude, ships, and spacecraft, and is far less useful for a soldier under tree cover. GAO listed it in 2021 among the absolute PNT sources DoD was pursuing.

Magnetic anomaly navigation

The earth's crust has a magnetic field that varies from place to place in ways that are mapped and stable over time. A sensitive magnetometer on an aircraft can measure those anomalies and match them against a map to estimate position, in the same way terrain-matching systems use elevation. Like celestial, it is passive and very hard to jam.

This area has moved quickly in public. The Department of the Air Force-MIT AI Accelerator ran a public Magnetic Navigation Open Challenge, built on an open-source software library, to improve filtering of aircraft magnetic noise, a core technical obstacle. That team, working with MIT Lincoln Laboratory, the Air Force Research Laboratory, and the Air Force Institute of Technology, then demonstrated real-time magnetic navigation on a C-17A during Exercise Golden Phoenix, which the Air Force described as the first operational demonstration of the technology on a DoD aircraft. In 2024, a C-17 from the 16th Airlift Squadron at Joint Base Charleston flew with SandboxAQ's magnetic navigation system, which uses quantum magnetometers and machine learning, as its primary navigation reference, with support from AFWERX and the National Geospatial-Intelligence Agency. According to Aviation Today's August 2024 report, three of five flight segments achieved required navigation performance of 1.0 nautical mile, and the system had logged more than 200 flight hours across more than 40 sorties on four aircraft types.

That is a meaningful result, and it also shows the current performance envelope: navigation-grade accuracy at the level of hundreds of meters to a nautical mile, not GPS-level meters. Performance depends on the quality and resolution of magnetic anomaly maps, which vary by region, and on how well the aircraft's own magnetic signature is modeled. Magnetic navigation is a strong candidate for bounding inertial drift on aircraft over land and water. It is less mature for ground vehicles, which sit close to large and variable local magnetic disturbances.

Vision-aided and terrain-referenced navigation

Cameras and other imaging sensors can estimate motion (visual odometry) and, with a reference database, absolute position by matching what they see to maps or imagery. Terrain-referenced navigation using radar altimeters and elevation databases has been used on cruise missiles for decades. The newer element is the availability of cheap cameras, onboard processing, and high-resolution reference imagery.

The war in Ukraine has pushed this hard. Heavy jamming and spoofing of GNSS on both sides has driven drone developers toward visual navigation, and a number of U.S. startups have publicly raised funding for GPS-free optical navigation aimed at small unmanned aircraft. Commercial performance claims are generally not independently verified in public.

Vision has clear limits: night, weather, smoke, featureless terrain such as open water or desert, seasonal change, and the need for current, accurate reference imagery. It also creates a data management problem. A vision system is only as good as the maps it carries, and those maps have to be produced, updated, distributed, and protected.

Quantum inertial sensors and atomic clocks

Every A-PNT design leans on inertial sensors and clocks to carry the solution between absolute fixes. Improving them stretches how long a platform can operate without an external reference.

Chip-scale atomic clocks. DARPA-funded work produced chip-scale atomic clocks that are now commercial products and appear in fielded A-PNT equipment. The Army has publicly listed chip-scale atomic clocks as one layer in its mounted A-PNT approach. They hold time far better than quartz oscillators over hours to days, which helps timing users and helps receivers reacquire signals.
Quantum inertial sensors. Cold-atom interferometers can, in principle, measure acceleration and rotation with much lower drift than conventional sensors. The challenge has been size, weight, power, ruggedness, and bandwidth outside the lab.
Quantum gravimeters and magnetometers. These support gravity-aided and magnetic navigation by measuring the environment more precisely.

The Defense Innovation Unit's Transition of Quantum Sensing (TQS) program, launched in 2024, is the clearest public view of where this stands. DIU announced in March 2025 that field testing was beginning, with more than ten tests planned across ground, air, and maritime domains in an initial 12-month phase, involving 17 performers working on quantum inertial sensors, gravimeters, magnetometers, and enabling components. Fielded quantum inertial navigation at scale in tactical platforms has not been publicly demonstrated, and claims beyond what DIU and the services have released should be treated with caution.

Comparing the options

The table below is a simplified summary of the main sources and their principal limits.

Source	Provides	Main strengths	Main limits
M-code GPS with protected antenna	Absolute position and time	Global, accurate, encrypted, and better anti-jam	Still a space-based signal that can be jammed; fielding delays
Commercial LEO PNT	Absolute position and time	Stronger signals, fast geometry change	Early maturity, commercial dependence, and jamming risk
eLoran and terrestrial beacons	Absolute position and time	High power, hard to jam over wide areas	Fixed infrastructure, not expeditionary, and limited U.S. coverage
Signals of opportunity	Absolute position, sometimes time	Signals already exist	Not under friendly control, needs transmitter knowledge
Celestial	Absolute position, attitude	Passive, unjammable	Weather, daylight, and platform motion
Magnetic anomaly	Absolute position	Passive, unjammable	Map quality, platform noise, and modest accuracy
Vision and terrain matching	Relative and absolute position	Passive, low cost sensors	Lighting, weather, featureless terrain, and map currency
Inertial (conventional and quantum)	Relative motion, attitude	Self-contained, unjammable	Drift over time; quantum still in testing
Atomic clocks	Time holdover	Self-contained, stable	Drifts eventually; cost and power for higher grades

The integration problem

The sources above are the easy part to describe. The hard part is integrating them so that a platform gets a trustworthy answer, and so that new sources can be added without redesigning the platform each time.

Sensor fusion and integrity

Fusion is usually implemented with some form of Kalman filter or related estimator that combines measurements weighted by their expected error. That works well when every sensor behaves as modeled. Contested environments break that assumption. A spoofed GPS signal, a vision system matching the wrong landmark, or a magnetometer reading a nearby vehicle can each inject confident, wrong data.

Good A-PNT fusion therefore needs more than a filter:

Fault detection and exclusion to identify which source disagrees with the others and remove it.
Integrity outputs that tell the user and downstream systems how much confidence to place in the solution, not just a position.
Threat awareness so the system recognizes jamming and spoofing as electromagnetic attack, rather than treating them as ordinary noise.
Operator notification, which is why DOT&E evaluated DAPS Gen II on both position accuracy and notification accuracy.

Testing this is difficult. Representative threat environments are hard to create on open ranges because of spectrum restrictions, and simulation must model not only signals but the behavior of every sensor in the fusion stack. DOT&E's finding that DAPS Gen II showed decreased position and notification accuracy under very challenging threat environments is a reminder that fusion performance under stress is the real measure.

Open architectures and standards

Federal law, at 10 U.S.C. 4401, requires major defense acquisition programs that received Milestone A or B approval after January 1, 2019 to be designed and developed, to the maximum extent practicable, with a modular open system approach (MOSA) to enable incremental development and enhance competition, innovation, and interoperability. For PNT, that matters because the right mix of sensors will change as threats and technology change. A closed, single-vendor box locks a platform into one fusion design for its service life.

Several standards and reference architectures are relevant:

VICTORY (Vehicular Integration for C4ISR/EW Interoperability) defines how vehicle subsystems share data, including standardized PNT data messages. The EW in the acronym dates from when "electronic warfare" was the term; current doctrine uses electromagnetic warfare. Under VICTORY, an A-PNT source can publish position and time to every consumer on the vehicle network rather than each subsystem carrying its own receiver.
CMOSS (C5ISR/EW Modular Open Suite of Standards), developed by the Army's C5ISR Center, combines VICTORY with the Sensor Open Systems Architecture (SOSA) technical standard, OpenVPX hardware, and related standards so that capabilities can be added as cards in a common chassis. Industry has publicly demonstrated A-PNT cards in the CMOSS mounted form factor that combine a chip-scale atomic clock, an inertial measurement unit, an alternative navigation receiver, and processing on a single 3U OpenVPX module, and that distribute timing and PNT data across the vehicle using VICTORY messages.
Plug-and-play fusion frameworks. DARPA's All Source Positioning and Navigation (ASPN) program, described publicly in 2012, set out to build an adaptable, plug-and-play sensor fusion architecture that accepts inputs from GPS, radio beacons, inertial units, and other sensors such as cameras and magnetometers. Program offices should confirm the current status of any ASPN-derived interfaces directly with the sponsoring organizations.

GAO's 2021 assessment listed institutionalizing open architecture standards as one of its policy options, alongside clarifying actual performance requirements and focusing on resilience rather than GPS replacement. Those three points remain the core of the integration problem.

Size, weight, power, and cost

Every additional sensor costs space, power, cooling, and money, and adds certification work. A tank can carry a large antenna array and a navigation-grade inertial unit. A small drone, a dismounted soldier, or a guided round cannot. The same A-PNT capability rarely fits across platform classes, which is one reason the Army runs separate mounted and dismounted programs and why MGUE Increment 2 exists for small form factors.

Data and maps

Magnetic, gravity, terrain, and vision-aided navigation all depend on reference data. That makes geospatial data production, accuracy, currency, classification, and distribution a part of the PNT problem. NGA's involvement in the C-17 magnetic navigation flights reflects this.

Timing users

Many systems use GPS only for time. Radios, networks, radars, and fires systems may lose timing during jamming even when nobody is trying to navigate. A-PNT programs that focus on position can leave timing users exposed. The CMOSS approach of making the A-PNT card a vehicle timing master, and the use of chip-scale atomic clocks for holdover, are direct responses to this.

What program offices should watch

GPS modernization milestones

OCX constellation transfer and operational acceptance. DOT&E's FY2025 report places the constellation transfer in FY2026, with operational acceptance afterward. Full M-code operational use depends on it.
First GPS IIIF launch. DOT&E lists FY2027. The regional military protection capability on IIIF is a significant improvement for contested theaters, and any slip affects plans that assume it.
MGUE Increment 1 lead platform testing on maritime and aviation platforms, and MGUE Increment 2 progress toward small form factor cards.
Receiver card supply. GAO's concern about a gap between legacy card production ending and M-code cards arriving is a real sustainment risk for platforms in production now.

Army A-PNT fielding and follow-on

The pace of MAPS Gen II and DAPS Gen II fielding against the total installed base of legacy receivers.
Results from units using these systems in training environments with realistic interference, especially reports on notification accuracy and operator trust.
Public detail on NorthStar and other modular follow-on efforts, which may indicate how the Army intends to add new sources without new programs of record.

Alternative sources moving out of the lab

DIU TQS results and whether any quantum inertial sensors, gravimeters, or magnetometers transition into service programs.
Expansion of magnetic anomaly navigation beyond demonstration flights, including pod-based and unmanned configurations.
Commercial LEO PNT constellation buildout, including the number of operational satellites, coverage, and any DoD agreements for military use.
DOT's complementary PNT findings, which will shape domestic critical infrastructure timing and indirectly affect DoD installations that rely on civilian infrastructure.
Decisions on Resilient GPS funding and architecture.

Requirements and acquisition practice

Write requirements against mission performance, not GPS performance. GAO's 2021 point stands: GPS-level accuracy everywhere is often not the requirement. Specify accuracy, integrity, continuity, and time holdover for the mission threads that matter.
Require integrity and notification outputs and test them, not just position accuracy.
Specify open interfaces such as VICTORY and CMOSS-aligned hardware where they apply, and government data rights to the fusion interfaces, so new sensors can be added later.
Account for timing users on the platform and in the network, not just navigation users.
Plan for reference data when selecting map-dependent sources, including who produces, updates, and distributes the maps.
Budget for test infrastructure. Realistic electromagnetic attack environments, including spoofing, are necessary to evaluate fusion performance, and range time with authorized interference is limited.

Bottom line

Assured PNT is a design discipline, not a product. GPS, and especially M-code GPS with protected antennas, remains the backbone, and the department is still years into delivering the full modernized system. GAO and DOT&E reporting through 2024 and 2025 shows real progress: M-code satellites on orbit, OCX delivered and in test, and Army A-PNT systems in full-rate production and reaching units. It also shows persistent schedule risk in the ground segment, user equipment, and receiver card supply.

The alternatives are real but uneven. Inertial sensors and chip-scale atomic clocks are mature and already fielded. Magnetic anomaly navigation and vision-aided navigation have strong public demonstrations. Commercial LEO PNT is early. Quantum inertial sensing is in field testing, not in service. eLoran remains a debated domestic timing option rather than a U.S. service. None of them replaces GPS, and none of them is immune to the problems of integration, reference data, and certification.

The programs that will do best are those that define what accuracy and integrity their missions actually need, build open fusion architectures that can take new sources over time, treat timing as seriously as position, and test against realistic electromagnetic attack.