The aviation industry has spent decades building ADS-B into the backbone of airborne surveillance. The technology works. For aircraft separated by miles of airspace, broadcasting position once per second over a shared 1090MHz frequency is a reasonable architecture. The problem is that airports are now equipping ground vehicles with the same system, and the physics that make ADS-B reliable in the sky work against it on the surface.
Understanding why requires looking at one design decision that rarely appears in procurement conversations: how ADS-B handles multiple transmitters sharing the same frequency.
The Protocol Was Built for Separation, Not Density
ADS-B operates on a random-access broadcast architecture, specifically a variant of the pure ALOHA protocol, as specified in RTCA DO-260B. When a vehicle or aircraft has a position update, it simply transmits. There is no coordination between transmitters, no "listen-before-talk" mechanism, and no assigned turn. In open airspace, where aircraft are separated by miles, the probability of two transmitters firing at the exact same microsecond is low enough that the system functions reliably.
On an airport surface, it is not.
The FAA's own position makes this explicit. Advisory Circular 150/5220-26, the FAA's governing document for airport ground vehicle ADS-B equipment, states that VMAT units "operate on either the 1090 ES link or the 978 MHz/UAT link. However, due to 1090 MHz spectrum congestion, the FAA strongly prefers the use of the uncongested 978 MHz/UAT link." The same document authorizes "a maximum of 200 (1090 ES and UAT) VMATs per airport to ensure no performance degradation of other FAA surveillance systems operating on the 1090 MHz frequency." Funding guidance goes further. Program Guidance Letter 24-01, issued October 2024, restricts AIP funding for VMAT deployments to a single link: "VMATs may only use the 978 MHz/UAT link." In a published advisory circular and in its funding guidance, the FAA has already picked which frequency it thinks ground vehicles belong on. Vendors that put ground vehicles on 1090 MHz are choosing the frequency the FAA caps for performance reasons and refuses to fund.
The failure mode is worth understanding. When two ADS-B signals overlap at the receiver, neither is decoded. At the protocol layer, the target does not appear with a degraded position or a warning flag. It simply disappears until the next successful transmission. For a vehicle moving at taxiway speed, a missed update cycle means the last known position is already wrong. Deployed ATC systems like ASDE-X add track coasting and multi-sensor fusion to compensate, but purpose-built ground vehicle tracking systems that rely solely on ADS-B receive no such safety net. The data gap is real, and for smaller airports operating without ASDE-X infrastructure, there is no fallback.
There is also a security dimension that rarely enters the procurement conversation. ADS-B is a public broadcast protocol. Its transmissions are unencrypted and readable by anyone with a standard receiver. ICAO acknowledged this as a structural vulnerability at its 12th Air Navigation Conference and has an active cybersecurity task force working on it. For airport security vehicles, maintenance operations in sensitive areas, or any movement data that should remain operationally confidential, this is a design limitation, not a configuration issue.
The Power Problem Compounds It
Aircraft transponders are required to transmit at a minimum of 125 watts on 1090MHz per FAA regulations (FAA AC 20-165), with commercial units commonly running 240 to 250 watts. Ground vehicle squitter units operate under a separate, lower-power FAA specification (FAA-E-3032).
The collision problem has been formally modeled. A 2015 peer-reviewed simulation by Van Der Pryt and Vincent, published in the journal Positioning, applied pure ALOHA mathematics to ADS-B packet loss across varying transmitter densities. At 100 targets, approximately 7% of packets are lost to signal collision. At 250 targets, that figure reaches 17%. At 300, it crosses 20%. Those numbers come from a wide-area satellite receiver scenario, not a compressed apron where vehicles operate within meters of each other. Ground-level density makes the collision problem worse, not better. Geometry compounds it. This is a classic near-far problem: surface vehicles sit orders of magnitude closer to ground-based receivers than the aircraft those receivers are tracking, so even with lower transmit power, surface signals can dominate the receive path and mask weaker airborne returns.
A large aircraft on the runway transmitting at full power will overwhelm the receiver at the same moment a ground vehicle's lower-power signal arrives. The ground vehicle loses that transmission slot. It is a foreseeable consequence of sharing one frequency between equipment built to different power specifications, in an environment where proximity magnifies the imbalance.
What a Deterministic Architecture Solves
The core issue is that ADS-B's random-access protocol was designed for an environment where low collision probability was a reasonable assumption. Ground vehicle tracking at operational density, and on the same frequency as high-power airborne transponders, violates that assumption in ways the FAA has formally acknowledged.
An alternative is a private radio network engineered specifically for the environment it serves. Rather than sharing a public frequency with every transponder on the movement area, a private 900MHz ISM network uses TDMA (Time Division Multiple Access) to assign each transmitter a dedicated time slot. There is no competition for the channel. No collision probability that rises with headcount. Whether there are 20 assets active on the apron or 200, each transmits in its assigned slot and the update rate does not degrade.
Because the network is owned and operated on-site, location data never routes through third-party infrastructure. Coverage gaps are solved by adding receivers on your timeline, under your control. The same network handles vehicles and personnel in a single operational picture, with the system actively alerting when something goes wrong: a vehicle breaching a restricted zone, a speed violation on a taxiway, two assets in dangerous proximity to each other, or a ground crew member approaching an active movement area without clearance. These are not passive position feeds. They are active safety interventions delivered in real time to both operators and the people in the field.
Unlike ADS-B's public broadcast architecture, a private RF network delivers an encrypted, confidential data link, keeping sensitive movement data on-site and inaccessible to outside receivers.
For airports evaluating this approach, RTS builds ground vehicle and personnel tracking on exactly this architecture: private 900MHz ISM radio with TDMA, delivered through the vMonitor platform. Data travels from tracker to base transceiver unit in approximately 50ms, consistent regardless of how many assets are active on the network.
What to Actually Evaluate
If a ground vehicle tracking system uses ADS-B as its communication layer, the questions worth asking before deployment are straightforward. What is the vehicle and personnel count on the movement area during peak operations? Does the vendor's system rely solely on ADS-B reception, or does it fuse additional data sources to compensate for packet loss? How does the system behave when a packet is lost? Does it alert, or does the target simply freeze? And what happens to your movement data once it leaves the transmitter?
The FAA's position is on record, in multiple forms. The agency that mandated ADS-B Out for commercial aviation has told airports that 1090 MHz is not its preferred link for ground vehicles, capped total VMAT deployments to protect other systems on that frequency, and refuses to put federal funding behind any deployment on 1090 MHz. Three separate policy signals, all pointing the same direction. That is worth weighing before specifying a system that puts every vehicle on the congested band.
ADS-B solved a real problem in airborne surveillance. Applying it to ground vehicle tracking at operational density is a different problem, and the communication architecture deserves the same scrutiny as the device on the dashboard.