The short answer
Aircraft tracking works mainly because aircraft broadcast their own position. A system called ADS-B sends an aircraft's identity, position, altitude and speed every second or so, and networks of ground stations, and some satellites, receive it. Satellite tracking works the other way round: satellites are observed from the ground, their orbits are published as orbital elements, and their future positions are calculated from the physics of orbits. That is how pass times and revisit windows are predicted.
For analysts, the two together answer practical questions: what flew over or into a site, and when the next satellite will be able to see it. Both have blind spots, from aircraft that switch off their broadcasts to satellites that maneuver or are missing from public catalogs.
How aircraft broadcast their position
Air traffic control has long relied on transponders: radios aboard an aircraft that reply when a ground radar interrogates them. Two kinds of transmission matter for tracking.
- Transponder replies. Older modes return a four-digit identity code and the aircraft's pressure altitude. The newer Mode S adds a unique address assigned to each aircraft, which ties every reply to one airframe.
- ADS-B. Automatic Dependent Surveillance-Broadcast goes further. The aircraft works out its own position from satellite navigation and broadcasts it, with identity, altitude, speed and heading, without being asked. Many countries now require it in busy airspace.
These broadcasts are not encrypted, and anyone with a suitable receiver can pick them up. Public flight-tracking services rely on large networks of ground receivers, many run by volunteers, and increasingly on receivers carried by satellites. When an aircraft's transponder replies but does not broadcast a position, its location can often still be worked out by multilateration: comparing the exact moments the same reply reaches four or more receivers. Air traffic control and military radars also see aircraft that broadcast nothing, but that picture is rarely public.
Why some aircraft are hard to see
- Transponders off or quiet. Military and other state aircraft are often exempt from broadcast rules and may fly with ADS-B or the transponder switched off, especially on operations. Some broadcast identity and altitude but not position. This is why military aircraft tracking is patchy on public services.
- Privacy programs. In some countries, owners of private aircraft can ask for their flights to be withheld from public displays, or use a temporary address that is not linked to the registered owner. Not every tracking service applies these requests.
- False identities and positions. Broadcasts are not authenticated. An aircraft can transmit a call sign that hides who operates it, and positions can be wrong because of faulty equipment, deliberate spoofing or jammed satellite navigation.
- Coverage gaps. Over oceans, deserts and mountains, or at low altitude far from receivers, an aircraft can be broadcasting with no one there to hear it.
For an analyst the rule is simple: the absence of a track is not the absence of an aircraft.
How satellites are tracked
Satellites carry nothing like ADS-B for public tracking, so they are watched from the ground. Military space surveillance networks follow objects in orbit with radar and telescopes, and the US publishes orbital elements for most of them: a compact description of each orbit's size, shape, tilt and position at a given moment. Many classified satellites are left out, although amateur observers follow a number of them on their own.
From those elements, standard models calculate where a satellite will be at any future time. The type of orbit sets the rhythm:
- Low Earth orbit. Most imaging satellites circle the planet roughly every hour and a half, a few hundred kilometers up, seeing a narrow strip of ground on each pass.
- Sun-synchronous orbit. Many Earth observation satellites cross the equator at the same local solar time on every orbit, so the lighting stays similar from one image to the next.
- Geostationary orbit. About 36,000 kilometers above the equator, a satellite keeps pace with the Earth's rotation and stays over one spot, as many weather and communications satellites do.
Satellite pass prediction and revisit windows
A pass is the stretch of time when a satellite is above the horizon at a given place. For an imaging satellite the question is narrower: when will the target be inside the area its sensor can reach? That depends on:
- The ground track the orbit traces across the Earth, which shifts with every revolution.
- How far the satellite can point to the side. Wider pointing means more chances to see a site, at a more oblique angle and with less detail.
- Light and weather. Optical sensors need daylight and clear sky; radar works at night and through cloud.
- How many satellites share the job. A constellation of many satellites shortens the wait between looks.
Predictions hold well for days ahead but drift as time passes since the orbital elements were published, fastest in low orbits where atmospheric drag is strongest. Any maneuver resets the picture.
What each kind of tracking tells you
| Aircraft tracking | Satellite tracking | |
|---|---|---|
| How position is known | The aircraft broadcasts it, or receivers work it out from its replies | Observed from the ground, published as orbital elements, then calculated |
| How often it updates | Every second or so while a receiver is in range | Elements are refreshed periodically; positions in between are predicted |
| How predictable it is | Follows a flight plan, but can change course at any moment | Follows physics, so it is predictable days ahead unless it maneuvers |
| How it can hide | Broadcasts switched off, privacy filters, a false identity | Left out of public catalogs, maneuvers, small size |
| What it tells an analyst | Who flew where, when and how often, and what breaks the usual pattern | When a place can next be seen from space, and by which sensor |
How analysts use it
- Activity around a site. Flights in and out of an airfield near a plant, a port or a base: cargo flights building up, unusual visitors, patrols along a coastline, tanker aircraft holding over one area. Departures from the usual pattern are what matter.
- Correlating air, orbit and ground. A rise in flights to a remote site, followed by new structures in satellite imagery, is stronger evidence than either on its own. The ground side of that picture is the subject of our guide to geospatial intelligence for business.
- Planning collection. Knowing when the next satellite can see a target, and with which sensor, tells an analyst when fresh imagery can exist, which matters to anyone using satellite data for investing.
- Knowing when you are watched. The same predictions show when your own facilities fall within view of other parties' satellites.
Limits and responsible use
- Gaps are normal. Coverage holes, switched-off transponders, privacy filters and catalog omissions all leave blanks. Report what was not seen as unknown, not as nothing.
- Data can be wrong. Identities can be false and orbital predictions stale. Cross-check anything that matters against a second source.
- Correlation is not cause. An aircraft over a site at the time of an event is a lead to follow up, not a finding.
- Respect privacy. Tracking business aircraft can reveal where individuals go. Honor privacy programs, follow the law where you operate, and use the data to analyze organizations and activity, not to follow private people.
- Handle sensitive operations with care. Publishing the live position of military or emergency flights can put people at risk. Keep sensitive findings among the people who need them.
How SKYWAY does it
SKYWAY: live airspace and orbit on one screen
SKYWAY shows every aircraft aloft: civil airliners, state aircraft and military flights, including the ones flying dark, silent or under a false identity. Position, heading, altitude and history sit on one screen, so the sky over any site reads as clearly as the ground under it.
It tracks the constellations overhead, commercial, civil and military, with their orbits, passes and revisit windows: when the next look at a given point on Earth will happen, and whose sensor will take it. Where a live feed exists, it shows the view from the platform itself, the camera, the radar or the sensor as it sees.
Every flying and orbiting asset sits in one index, with the operator, the mandate and the sensor payload declared for each, held to the same standard of evidence whether it belongs to an airline, an agency or a ministry. Behind the screen, 105 proprietary algorithmic layers run continuously against the air and orbit picture, reading events on the ground and above it as they happen, correlated to the MITHRIL ground picture.
Questions
What is ADS-B?
ADS-B, Automatic Dependent Surveillance-Broadcast, is a system in which an aircraft works out its own position from satellite navigation and broadcasts it, with its identity, altitude and speed, every second or so. Ground stations and some satellites receive these broadcasts, and most public flight tracking is built on them.
Why do some military aircraft not appear on flight trackers?
Military aircraft are often exempt from broadcast requirements and may switch ADS-B or their transponders off, or broadcast without a position. Some can still be located by multilateration when several receivers pick up their transponder replies.
How are satellite passes predicted?
Satellites are tracked from the ground and their orbits are published as orbital elements. Standard models use those elements to calculate where each satellite will be at any future time, and so when it will be above the horizon, or within sensor reach, of a given place.
How accurate are satellite pass predictions?
Accurate enough to plan around for days ahead, but errors grow with time since the elements were last updated, fastest in low orbits where drag is strongest. A maneuver makes older predictions unreliable.
Can you tell when a satellite will next image a location?
Within limits, yes. A satellite's predicted orbit and how far its sensor can point give the windows when a site is in reach; whether an image is taken depends on the operator and, for optical sensors, on daylight and cloud.
