Satellite Tracker

How this satellite tracker works

Every amateur satellite is described by a two line element set, a TLE, which is a compact snapshot of its orbit at a given moment. This site downloads the TLEs for the amateur satellites once every 24 hours from CelesTrak and stores them on its own server. When you open this page, your browser fetches that stored copy and runs the SGP4 orbital model locally to work out where each satellite will be over the next 24 hours from your position.

Nothing about your location is sent anywhere. Your Maidenhead locator stays in your own browser, the orbital arithmetic happens on your device, and the only thing this site provides is the orbital data itself. That also means the tracker keeps working normally if you have a slow connection, because the heavy part is not a server round trip.

Orbital elements decay in accuracy as they age. A TLE that is a day old is normally good to within a second or two of pass timing for a low orbit satellite, which is far tighter than the tolerance you need to hear a bird. The status line at the top of the tracker tells you how old the current data is, so you always know what you are working from.

The 19 satellites this tracker follows

These are working satellites with transponders or digipeaters that are usable today. Frequencies are taken from the AMSAT live status pages. Uplink is what you transmit on, downlink is what you listen on.

Satellite	Mode	Uplink MHz	Downlink MHz	Notes
ISS	FM voice	145.990	437.800	CTCSS 67.0. The repeater is not always switched on.
SO-50	FM	145.850	436.795	CTCSS 67.0. Needs a 2 second 74.4 Hz tone to arm the timer.
AO-91	FM	435.250	145.960	Do not use it while the satellite is in eclipse.
AO-123	FM	145.850	435.400	CTCSS 67.0.
PO-101	FM	437.500	145.900	CTCSS 141.3. Ageing, and reported to be degrading.
TEVEL2-1 to 2-9	FM	145.970	436.400	Nine identical satellites on the same pair of frequencies.
RS-44	Linear, inverting	145.935 to 145.995 LSB	435.610 to 435.670 USB	Wide, strong and busy. The easiest linear bird to start on.
AO-7	Linear, inverting	432.125 to 432.175 LSB	145.925 to 145.975 USB	Mode B. Launched in 1974 and still working, but only in sunlight.
FO-29	Linear, inverting	145.900 to 146.000 LSB	435.800 to 435.900 USB	Currently in full sunlight.
AO-73	Linear, inverting	435.130 to 435.150 LSB	145.950 to 145.970 USB	Transponder stays active in eclipse.
JO-97	Linear, inverting	435.100 to 435.120 LSB	145.855 to 145.875 USB	Transponder works. Telemetry is dead.
IO-117	Digital digipeater	435.310	435.310	Same frequency up and down. High orbit, so a very wide footprint.

How to work your first satellite

Start with an FM satellite and start by listening. Pick a pass in the list above with a maximum elevation of 30 degrees or more, because a high pass gives you a longer window and a stronger signal than a pass that only grazes the horizon. Set your radio to the downlink frequency a few minutes before AOS and simply listen. On a good pass you will hear stations calling long before you need to transmit anything.

An FM satellite is a single channel repeater moving at roughly 7 kilometres per second, so only one contact happens at a time and the exchange is short: both callsigns and a grid square. A handheld with a small directional antenna is enough. The two things beginners get wrong are transmitting on the downlink frequency, which does nothing, and forgetting to correct for Doppler, which makes you disappear halfway through the pass.

Linear satellites such as RS-44 are quieter and more forgiving, because they carry many simultaneous contacts in a passband rather than one at a time. They need SSB rather than FM, and they need you to tune continuously, but they reward patience and a modest station.

Understanding Doppler shift

A satellite in low Earth orbit approaches you at several kilometres per second, passes overhead, then recedes at the same rate. That motion compresses the signal on the way in and stretches it on the way out, so the frequency you actually receive is not the frequency printed in the tables. The shift is proportional to frequency, which is why it matters far more on 70 centimetres than on 2 metres.

On a 435 MHz downlink the total swing across a pass is roughly plus and minus 10 kHz. At AOS the satellite is coming towards you and the signal appears high in frequency; at the moment of closest approach the shift passes through zero; by LOS it is as far low as it was high at the start. On a 145 MHz downlink the same pass produces roughly a third of that swing.

The tracker does this arithmetic for you. The frequencies shown in the detail panel are already corrected: the downlink figure is where the signal will actually appear, and the uplink figure is where you should transmit so that your signal arrives on the satellite’s real receive frequency. If you have a radio with computer control you can follow those numbers continuously; if you are tuning by hand, adjust in small steps and follow the signal.

Reading the sky track

The circular plot in the detail panel is a view of your sky looking straight up. North is at the top and east is to the right. The outer edge is your horizon at 0 degrees elevation and the centre is directly overhead at 90 degrees, so the two rings mark 30 and 60 degrees. The orange line is the path the satellite will take across your sky, the blue dot is where it rises and the red dot is where it sets. If the pass is happening right now, a green dot shows the satellite’s current position.

A track that stays near the outer edge is a low pass: short, weak and often blocked by buildings or terrain. A track that crosses close to the centre is a high pass, and those are the ones worth planning your day around.

Frequently asked questions

What is a Maidenhead locator and how do I find mine?

A Maidenhead locator is a short code that describes a rectangle on the Earth’s surface, for example IO91 or IO91wm. Four characters give you a box roughly 100 by 150 kilometres, which is precise enough for satellite pass prediction. Six characters narrow it to a few kilometres. If you do not know yours, use the “Use my location” button and the tracker will work it out from your device.

Which satellite should I try first?

SO-50 is the usual answer for a first FM contact, because it is reliable, well known and worked by a large and patient community. For a first SSB contact through a linear transponder, RS-44 is the easiest: its passband is wide, its signal is strong, and passes are long because it sits in a high orbit.

Do I need a beam antenna to work satellites?

No, but it helps a great deal. Many operators make their first satellite contact with a handheld and a small hand held Yagi that they point manually. A vertical will hear the stronger satellites on a high pass, but it will struggle at low elevations. What matters more than gain is having a clear view of the sky in the direction the satellite is coming from.

What do AOS and LOS mean?

AOS is acquisition of signal, the moment the satellite rises above your horizon. LOS is loss of signal, when it sets again. The time between them is the whole of your opportunity, and for a low Earth orbit satellite it is typically between 5 and 12 minutes.

Why does maximum elevation matter so much?

Maximum elevation is how high the satellite climbs in your sky at its closest point, and it is the single best predictor of whether a pass is worth working. Below about 10 degrees the signal has to cut through a long slant path and whatever is on your horizon, and passes are short. Above 30 degrees the satellite is close, loud and in view for several minutes. That is why the tracker shows maximum elevation as the first number on every row.

What is an inverting linear transponder?

A linear transponder receives a slice of spectrum and retransmits it on another band. In an inverting transponder that slice is flipped, so moving your transmit frequency up moves your signal down within the downlink passband, and a signal sent as LSB comes back as USB. Most of the amateur linear satellites are inverting, which is why the tracker states it explicitly for each one.

Why does SO-50 need a 74.4 Hz tone?

SO-50 keeps its transmitter off until someone arms it. Sending a 74.4 Hz subaudible tone for about two seconds starts a ten minute timer, after which the repeater is usable with the normal 67.0 Hz tone. The timer can be restarted during the pass. This is a quirk of that satellite and does not apply to the others.

Can I work a satellite with a handheld radio?

For FM satellites, yes. A dual band handheld that can transmit on one band while receiving on the other, together with a small directional antenna, is a complete satellite station. Linear satellites need SSB, which most handhelds cannot do, so those require a different radio.

What is full duplex and do I need it?

Full duplex means hearing the downlink at the same time as you transmit on the uplink. It is not required, but it changes the experience completely: you can hear your own signal coming back through the satellite, which tells you immediately whether you are on frequency and getting in. Without it you are transmitting blind and correcting Doppler by guesswork.

What is a TLE and why does it expire?

A two line element set is a compact description of an orbit at one instant, using the mean elements that the SGP4 model expects. It is not a fixed truth: atmospheric drag, solar activity and small manoeuvres all move a satellite away from the prediction. Accuracy is good for a few days and best within the first day, which is why this tracker refreshes its data every 24 hours.

How often is the orbital data updated?

Once every 24 hours, by a scheduled job on this server that fetches the amateur satellite elements from CelesTrak. The age of the current data is shown at the top of the tracker. If a fetch fails, the last good set is kept and served rather than leaving you with nothing.

Is my location sent to the server?

No. Your locator is stored in your own browser and the pass calculations run on your device. The only request this page makes is for the orbital elements, which are the same for everyone.

Why do some passes show only 2 or 3 degrees elevation?

Because they are real, geometrically. The tracker lists everything that clears a mathematical horizon, and a 3 degree pass genuinely exists even though most stations will hear nothing. Terrain, buildings and trees usually take the practical horizon up to 5 or 10 degrees. Treat low elevation passes as a bonus rather than a plan.

Do I need a licence to listen?

No. Receiving amateur satellites needs no licence anywhere that listening to amateur radio is lawful, and listening is the right way to start. You need an amateur licence to transmit, and you must work within the privileges and power limits of your own licence class.

Why are there nine TEVEL satellites on the same frequency?

The TEVEL constellation was built and launched together by a group of Israeli schools, and all nine carry identical FM transponders on the same pair of frequencies. In practice they are spread around their orbit, so usually only one is in view at a time. When two are up together they will interfere with each other, and the tracker lists them separately so you can see which is which.

Where the data comes from

Orbital elements come from the amateur satellite group published by CelesTrak, maintained by Dr T.S. Kelso, which is the reference the whole hobby uses. Frequencies, modes and operational status come from the AMSAT live satellite pages. Pass geometry is computed with the SGP4 model through the satellite.js implementation, running in your browser.

Satellite status changes. Transponders are switched off for eclipse management, batteries age, and satellites are retired. If something here looks wrong, check the AMSAT status page and please get in touch so it can be corrected.

More tools on this site

If you are chasing a satellite pass you are probably also watching conditions elsewhere. The HF propagation score gives a single figure for how the bands are behaving right now, the live DX cluster shows where stations are actually being heard, the beacon monitor tells you which NCDXF beacon is transmitting at this second, and the contest calendar lists what is on this weekend.