Jet-powered UAS
An illustrative example of a lost link on a jet-powered target UAS, rebuilt from the onboard Chapter 10 recording and the range's own records. It is worked from published standards, and it is not a real event.
On this page8
Illustrative example
This page walks through one illustrative example. It is not a real event, and it is not in the console's example dataset.
The aircraft, test point, link budgets and lost-link logic are design values worked from published standards: RCC 106-23 (Chapters 2, 4, 10 and 11), RCC 319-25 and FAA JO 7110.65BB 5-2-6. The belly antenna's shadow curve is an assumption, and the page labels it as one.
A jet on a test range loses both links for a few seconds, and its flight computer enters lost-link logic. The review board asks one question: did the aircraft decide, or did the ground lose it first?
In this illustrative example, a 5 g reversal 110 km from the ground site hides the belly antennas from the ground. The lost-link timer fires while the ground is blind, and the reconstruction shows it fired exactly on time.
The aircraft and the test point#
Illustrative example| Item | Value |
|---|---|
| Aircraft | Turbojet target-class UAS, 640 kg at the event, 4.2 m span |
| Test point | TP-7: 5.0 g level turn left, 180° reversal, FL150, M0.70, 110 km due east of the ground site |
| Speed and roll | 225.59 m/s true airspeed, M0.70 at FL150 in the standard atmosphere, rolling at 60°/s |
| Bank | 78.46°, the bank of a 5 g level turn: arccos(1/5) |
| Lost-link logic | No valid uplink frame for 3.0 s: abort the test point, roll wings level, climb to FL160 and fly to the lost-link orbit |
| Ground site | Telemetry (TM) and command and control (C2) site due west, 2.75° below the aircraft's horizon on a 4/3 earth |
| C2 antenna | Belly only |
The link budget sets where each link fails. With a clear line of sight, TM arrives at the ground at −76.2 dBm and the C2 uplink at the modem at −87.2 dBm. Both lose lock at −105.0 dBm.
| Level | What it marks |
|---|---|
| −76.2 dBm | TM received at the ground, clear line of sight |
| −87.2 dBm | C2 uplink received at the modem, clear line of sight |
| −99.5 dBm | TM at a bit error rate of 1e-5 (Eb/N0 12 dB) |
| −103.0 dBm | TM at a bit error rate of 1e-3 |
| −105.0 dBm | Decommutator lock and modem sensitivity |
The example assumes the belly antennas lose nothing up to 15° above the belly plane, then 1 dB per degree, capped at 32 dB. On that assumption, the uplink falls below the modem's sensitivity once the line of sight to the site passes 32.8° above the belly plane. The ground loses TM frame lock at 43.8°.
Sources read#
Illustrative example| Source | Where it is recorded | What it adds | Format |
|---|---|---|---|
| Onboard IRIG 106 Chapter 10 recording | The aircraft's recorder | Time, PCM, MIL-STD-1553, ARINC 429 and Ethernet channels: flight computer status, attitude, the C2 modem's valid-frame counter and the FTS monitor words | IRIG 106 Chapter 10 |
| Ground telemetry recording and receiver log | The ground station | The ground's view of the downlink, including the decommutator's frame lock | IRIG 106 Chapter 10, range records |
| C2 ground station log | The C2 ground station | What the operator sent, and when | No format page |
| Radar TSPI | The range | A track of the aircraft measured from the ground, every 50 ms | Range records |
| FTS console log | The range | Commands sent from the flight termination console | Range records |
IRIG 106 Chapter 10 recordings lists each channel of the example recording by data type.
What happened#
Illustrative exampleTimes are vehicle time in UTC, starting from 17:42:03.000. Ground stamps are already corrected by a measured pipeline offset of 2.270 ms.
| Event | Vehicle time, UTC | Source | What it shows |
|---|---|---|---|
| A1 | 17:42:03.000 | Recorder, 1553 FCC status | The flight control computer (FCC) starts TP-7: roll into a 5 g left reversal at 60°/s |
| A2 | 17:42:03.580 | Recorder, C2 modem valid-frame counter | Last valid uplink frame |
| A3 | 17:42:03.779 | Ground Chapter 10 PCM, LOCKST | Decommutator loses frame lock |
| A4 | 17:42:06.580 | Recorder, 1553 FCC status | LOST LINK, 3.000 s after the last valid frame; roll-out begins |
| A5 | 17:42:06.861 | Ground Chapter 10 PCM, LOCKST | Frame lock regained; the first frames already carry LOST LINK |
| A6 | 17:42:07.140 | Recorder, C2 modem valid-frame counter | First valid uplink frame; the FCC stays in LOST LINK, as designed |
| A7 | 17:42:07.888 | Recorder, EGI attitude; radar TSPI | Wings level on 324.6°, then a 30° right turn toward the lost-link orbit |
| A8 | 17:42:10.228 | PCM FTS ARM discrete; FTS console log | Flight termination armed as a precaution, 12.0 ms after the console command |
| A9 | 17:42:12.941 | Recorder, 1553 FCC status; C2 ground log | Operator RESUME clears LOST LINK 37 ms after it was sent |
A10 follows at 17:42:38.532, when the range safety officer removes the arm tones.
The uplink was out for 3.560 s, from A2 to A6. The ground had no TM frame lock for 3.082 s, from A3 to A5. By the time LOST LINK rolled the wings level, the aircraft had flown 35.4° of the planned 180° reversal.
The FCC's mode moves through three states: test point from A1, LOST LINK from A4 and resumed from A9. The FTS reads Safe until A8 and ARM after it.
The finding#
Finding: the test card, not the aircraft
Had the turn continued, the same geometry returns the uplink after 4.55 s, against a 3.0 s timer. With the site on the inside of a 5 g turn at 110 km and a belly-only C2 antenna, the lost-link entry was certain. Fly the reversal with the site on the outside, add a top C2 antenna or raise the timer for high-bank test points.
That answers the board's question. The ground lost the aircraft first: the uplink at A2 and TM frame lock at A3. The aircraft decided 3.000 s after the last valid frame, at A4, exactly as its lost-link logic specifies.
Why the clocks are not the limit here#
On the drone and ground robot examples, clock bounds limit the order of events. Here the recorder, the ground station and the range all run on IRIG-B, so their clocks agree to microseconds.
What limits the order is how often each source samples. At the example's 225.59 m/s, each sampling interval is also a stretch of flight path:
| Source | Sampling | Distance at 225.59 m/s |
|---|---|---|
| PCM telemetry | 1 ms frames | 0.23 m |
| MIL-STD-1553 status words | 20 ms bus schedule | 4.5 m |
| Radar TSPI | 50 ms samples | 11.3 m |
The FCC's status word, which carries LOST LINK, rides the 20 ms bus schedule. The report therefore places LOST LINK to ±20 ms, a limit set by sampling, not by any clock. Inside one recording, every channel shares a single counter, as the relative time counter explains.
What the evidence shows#
Illustrative example- The uplink and telemetry losses start 46 ms and 138 ms after the line of sight climbs past 30° above the belly plane. Both end as it falls back in the roll-out.
- LOST LINK engaged 3.000 s after the last valid uplink frame, exactly as configured, and the aircraft flew the lost-link route.
- The flight termination link never came near its margin. Its signal strength, read from PCM at 1 kHz, sat at 3.63 V and rippled between 3.42 and 3.84 V while the bank was above 45°. The lowest reading, 3.42 V, stayed far above the 1.66 V line of the 12 dB margin.
What it does not show#
- That the airframe caused the shadowing. The losses line up with the aircraft's attitude, so the shadowing is Correlated. The 15° knee and the 1 dB per degree slope are a Hypothesis that only an antenna pattern measurement can confirm.
- The exact instant the flight computer set LOST LINK. Status words ride a 20 ms bus schedule, so the report places it to ±20 ms.
- Why termination was armed. The console log records the command. The rule behind it lives in the range's procedures.
Related#
- IRIG 106 Chapter 10 recordings covers the packet format, the relative time counter and the setup record.
- Range records covers radar TSPI, the telemetry receiver log and the FTS console log.
- Clocks, anchors and error bounds turns a clock bound into distance at speed.
- Platforms compares the jet with the drone and ground robot examples.