Passive radar (experimental)

An FM radio station lights up the sky around it all the time. Aircraft reflect a tiny part of that broadcast. With two radio receivers, DroneTracker can compare the echoes with the station’s own signal and measure two things for every reflecting object at once:

• Bistatic range: how much further the echo travelled than the direct signal.
• Doppler: how fast that extra distance is changing, which is the object’s speed along the echo path.

Nothing is transmitted. The radar measures range and speed; it never names an object. Naming stays with the camera and with aircraft transponders (ADS-B).

Operators watch the radar, read-only, on the RF view’s Passive radar tab (The RF view). This page is where it is set up.

Experimental. The processing is built and tested on simulated signals, but it has not yet been proven on a real station. Expect aircraft and helicopters rather than small drones: at FM wavelengths (about 3 m) a small quadcopter reflects very little.

What you need

• Two RTL-SDR dongles plugged into the rig, besides any already used for ADS-B or airband. Ordinary dongles work: DroneTracker lines them up on the broadcast itself every second, so they do not need a shared clock. Two dongles modified to share one clock, or a phase-coherent receiver, also work.
• Two FM antennas (88–108 MHz):
  • the reference antenna points at the transmitter, to hear a clean copy of the station;
  • the surveillance antenna points at the part of the sky you want to watch, ideally with the transmitter behind it or blocked by a building or the terrain.
• No filter that removes FM: here FM is the signal.

Choosing a station

Admin → Passive Radar → Illuminators lists licensed FM stations near the rig from the FCC’s station list, best first. A station scores higher the more power it radiates and the closer it is. A station marked Too close (under 5 km) may overload the receivers. Choose Use on a row to pick it.

The rig needs its own location for this (Operator Location, or the location under Admin → Hardware). Without it the list is empty.

Setting up the receivers

Under Receiver:

1. Leave Receiver on Two dongles. Choose which dongle is the reference and which is the surveillance. Dongles already used for ADS-B, airband or datalink are marked; a dongle given to the radar stops doing its other job.
2. Leave Clock on Independent clocks for ordinary dongles.
3. Auto gain is fine for a first look. For real use, set a fixed gain on each channel: automatic gain can change during a second, which spoils the cancellation. Start around 20 dB. If the status shows clipping above 1 %, lower that channel’s gain.
4. Choose Save receiver. The rig restarts its radio service with the new assignment.

To give the dongles back, clear both selections and save.

The radar stays off until Run passive radar is turned on under Radar processing, below the receiver. The other settings there are for tuning against recordings; the defaults are a sensible start.

Antenna array (3–8 dongles)

With three or more dongles, the receivers can work as one antenna array. The array adds two things:

• A bearing for every echo. Each echo reaches the antennas at slightly different moments; the array turns that into the direction it came from. With the range, that places a radar track on the map.
• A null on the station. The array removes the transmitter’s own direction before the cancellation starts, so less of the direct signal is left to hide the echoes.

What you need:

• Three to eight dongles. Dongles sharing one clock, or a phase-coherent receiver such as a KrakenSDR, are the dependable choice. Ordinary dongles with their own clocks may work, but that is not proven yet; the status shows how steadily each one follows the broadcast.
• Identical FM antennas (simple dipoles are fine), mounted the same way up, in a circle or a straight line. Keep neighbouring antennas no more than about 1.5 m apart: half a wavelength at FM. Wider spacing can show one echo at several bearings.
• Optionally one more antenna aimed at the station as a separate reference. Without it, the array forms its own reference by listening toward the station.

Cable lengths do not need to match: every second the rig measures each channel against the station’s own signal and evens them out. That is also why it needs the station’s position: the station is what the array is calibrated on.

To set it up, under Receiver:

1. Set Receiver to Antenna array (3–8 dongles).
2. Choose a dongle for each channel; Add dongle and Remove change how many. With a separate reference, the first channel is the Reference antenna; the others are Element 1, Element 2 and so on, in order round the circle or along the line.
3. Set one fixed gain, dB for every dongle (start around 20).
4. Describe the array:
   • Reference: Separate antenna aimed at the transmitter, or Every antenna is an array element (the array forms its own reference by listening toward the station).
   • Layout: Circle or Straight line.
   • Radius (m) for a circle, or Spacing between neighbors (m) for a line. The form shows how far apart neighboring antennas are against half a wavelength at the chosen station, and warns when they are too far apart.
   • Bearing of element 1 from the center for a circle, or Bearing the line points along (element 1 at the back end) for a line. Use true north, not magnetic north.
   • Numbering, seen from above: Clockwise or Counterclockwise (circle only).
   • Spatial null: Toward the transmitter removes the station’s direction. Strongest signal or Two strongest signals remove the strongest directions measured from the signal itself: the station plus its strongest reflections off hills and buildings. Off keeps everything.
5. Choose Save receiver. The dongles you choose stop serving ADS-B, airband and VDL2.

A line of antennas cannot tell its two sides apart: an echo from one side looks the same as one from the mirror side. Those bearings are marked with ?, and their tracks are not placed on the map. Use a circle when you can.

If the radar’s tracks all sit turned by the same angle from the aircraft they match, the bearing of element 1 (or of the line) is off by that angle: correct it and save.

Reading the status

• Alignment: how far apart the two dongles’ streams and tuners are, and how steadily they follow the broadcast (coherence, 0–1). Coherence that stays high means the two receivers are working as one.
• Cancellation: how much of the direct signal and nearby ground reflections were removed, in dB. More is better. The echoes are about 90 dB weaker than the direct signal.
• Broadcast barely removed: a note in amber while cancellation is under 10 dB. Until it reaches about 20 dB, marks near zero range and zero Doppler are the broadcast leaking through, often as a mirrored pair at the same range, not objects. Cancellation can remove at most the part of the broadcast both channels share, and coherence measures that share: at 35 % coherence it is about 0.6 dB. Low coherence almost always means the reference channel hears the station weakly. Use an FM antenna aimed at the station for the reference (an ADS-B antenna is far too short for FM), and raise the reference gain while its clipping stays at 0 %.
• Clipping: the share of samples at the receiver’s limits for each channel. Above 1 %, lower that channel’s gain.
• Detections, tracks and ADS-B matched: what was found this second, how many tracks have lasted several seconds, and how many aircraft with ADS-B were found where they should be.
• Interference lines: a note in amber when one Doppler is raised at every range. That is a fault on a receiver, not echoes, and the radar keeps it out of its detections. A line at twice the mains frequency (120 Hz in North America, 100 Hz elsewhere) is almost always power-supply ripple reaching a dongle: power the dongles from a powered USB hub on a clean supply, keep their cables away from power adapters, and check again.
• Array (antenna array only): how much the null removed, in dB, and for each element how steadily it follows the broadcast, its strength and its start offset. Alignment above then shows the weakest element. An element whose coherence stays low is the one to check: its antenna, cable or dongle.

The range-Doppler view

Range runs along one axis and Doppler along the other. Brighter cells are stronger echoes.

• Amber circles are detections.
• Amber squares are tracks; they are filled once confirmed.
• Green diamonds show where aircraft with ADS-B should appear, labeled with their callsigns.

A bright vertical stripe through every range at one Doppler is an interference line (see Interference lines above), not a row of targets.

An aircraft found where its diamond is shows the radar is working. A confirmed track with no diamond nearby is something without a transponder. That is worth a look with the camera.

Without receivers, the view still shows the green diamonds: where the radar would see each aircraft from the chosen station.

The bearing scope (antenna array)

With an antenna array, a round scope shows the sky around the rig from above, north up. Distance from the centre is distance from the rig.

• Grey dots are aircraft with ADS-B; green ones are matched by the radar this second.
• Amber marks are radar tracks and detections that have a bearing.
• A line marks the direction of the station.

The detection and track lists show each bearing too: — without one, a ? after it when the mirror side fits as well.

Recordings

Record saves the next 1, 5 or 15 minutes of both receivers’ raw signal, with the ADS-B aircraft heard at the same time. Files use the open SigMF format. Each minute takes about 60 MB, and recording will not start with less than 2 GB free. Download the three files of a recording from the list, or delete it. Recordings are how the radar is tuned and checked, so keep a few from different times of day.

Track history

Every confirmed radar track is kept as one short record: when it was seen, its closest range, its last speed, its bearing and map position when an antenna array measured them, and the ADS-B aircraft it matched, if any. The raw maps are never kept. Ask the rig reads these records, groups the tracks an interference line left (many at one speed, at many ranges, at the same moment) and reports them as one fault rather than as objects, and answers questions like “anything on radar in the last hour?”. Records are kept 30 days; change Keep radar track history (days) under Admin → System Settings (search for “radar”).