SimTX Guide/Satellites

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A satellite is a station in orbit, designed and launched by a SimTX user. Every satellite belongs to an account; there are no built-in ones. Like a ground station it has antennas and radios, and many sessions can use it at the same time.

A satellite is sealed at launch. Its orbit, antennas, radios and attitude can't be changed afterwards; only the name, callsign and visibility can. To change anything else, deorbit it and launch a new one.

Launch a satellite

In the Stations tab, select Mine and press Launch satellite. The Launch a satellite workspace opens beside the globe, which shows the orbit as you design it. There are three steps: Orbit, Payload and Launch. Use Next and Back to move between them.

Orbit

Choose a starting point:

Preset	Orbit
Space station	Low orbit at 420 km and 51.6° that passes over most of the world.
Sun-synchronous	Crosses each place at the same local time every day.
Geostationary	Stays over one longitude, about 35,800 km up.
Custom	Set altitude, shape and tilt yourself.

More orbits adds Polar, Equatorial and Molniya. Changing a value that a preset fixes turns the design into Custom.

The controls describe the orbit in plain terms:

Altitude: how high the satellite flies, from 200 to 40,000 km. A higher satellite moves more slowly, sees a larger area and stays in view longer, but is farther away, so its signal is weaker. Choose Circular for one altitude, or Elliptical to set the Lowest point and Highest point and whether the highest point is over the North or the South.
Inclination: the tilt of the orbit against the equator. At 0° the satellite stays over the equator; at 90° it passes over both poles. Above 90° it moves against the Earth's rotation. The track reaches north and south up to the latitude of the inclination (or 180° minus it).
Sun-synchronous: the app sets the inclination for the altitude, slightly more than 90°, so that the satellite passes each place at the same local time.
Geostationary: at about 35,800 km over the equator the satellite turns with the Earth and stays fixed in your sky. Set the longitude under Parked over longitude. From a given place it is either always in view or never.
Molniya: a long, elliptical orbit at 63.4° whose high point hangs over high northern latitudes for hours on each orbit. Set the longitude under Highest point over longitude.
Starting point: where the first orbit begins, as latitude and longitude, heading Northbound or Southbound. Pick on globe places it by clicking the globe; Over my station… starts it over one of your ground stations.

Orbital elements shows the orbit as numbers and lets you type them directly. Back to guided controls returns to the controls above.

The figures beside the form update as you design: Orbit takes (the period), Speed, Footprint radius (how far from the point below it the satellite is in view), Max Doppler, Coverage, and how often it passes over each of your ground stations. If the orbit can't be flown, for example because the lowest point is too low, a warning says why.

Payload

Antennas: one to four. Satellite antennas are fixed to the body and work in free space; Rotation on the body turns an antenna on the body.
Radios: up to eight, each with a Power cap. Small satellites run 1 to 10 W. Link check estimates how strong the satellite would be at a ground station at 30° elevation on 437.8 MHz.
Attitude: how the satellite holds itself. Earth-pointing keeps one face toward the ground, with a Yaw angle. Spinning spins for stability, sweeping its antenna pattern past you; set the Spin rate and what it Spins around. Fixed in space holds one direction among the stars.
Name and visibility: the satellite's Name and Callsign, and Public or Unlisted. A public satellite is shown on the map, and anyone signed in can receive from it. An unlisted satellite can only be used by you, though others still hear it transmit.

Launch

Check the summary. Each line has an Edit link back to its step. Tick Sealed after launch and press Launch satellite. The new satellite's details open.

Satellite details

Select a satellite in the Stations tab to see:

Right now: altitude, speed, the point it is over, footprint radius and how long an orbit takes;
On this station: the sessions running on it;
Passes and Doppler now (see below);
its antennas and radios.

Edit changes only the name, callsign and visibility.

Passes

A pass is the time a satellite is above your horizon. It starts when the satellite rises (AOS, acquisition of signal) and ends when it sets (LOS, loss of signal). The elevation is how high above the horizon it is, from 0° at the horizon to 90° straight overhead. A pass with a high maximum elevation is longer and closer, so the signal is stronger. A pass that stays a few degrees above the horizon is short and weak.

In the satellite's details, the Passes panel lists the next passes over one of your ground stations, chosen under Seen from. Each row shows the time in UTC, the highest elevation and the direction it crosses the sky. Select a pass to see a sky plot and when it Rises, is Highest and Sets, how long it Lasts, and its Doppler range. For a geostationary satellite the panel says whether it is always in view and where to point.

Passes also appear elsewhere:

the Mine list shows the next pass of each satellite over your first ground station;
a ground station's details list the Satellites over this station;
in the Transmit and Receive tabs, choosing a satellite under Track a satellite shows the next pass.

All times are UTC in simulation time, the clock shown in the map legend.

Passes are predicted against a flat horizon. Hills and mountains around a station can hide a satellite that is low in the sky, so a low pass, or the first and last minutes of a pass, may be silent.

Doppler

A satellite moves at several kilometres per second. While it comes toward you its signal arrives higher in frequency, and while it moves away, lower. On 70 cm the shift reaches several kilohertz during a pass. The Doppler now panel in the satellite's details shows the current shift for a frequency you enter.

To correct it, choose the satellite under Track a satellite in the Station section of the Transmit or Receive tab. Correct Doppler turns on with it:

when receiving, the tuning follows the shift, so the satellite stays at the same place in your SDR program;
when transmitting, the frequency is shifted the other way, so the signal arrives on frequency at the satellite.

On a ground station with a rotator, choosing a satellite also makes the antenna follow it; see SimTX Guide/Rotators and tracking.

Transmit from or receive at a satellite

A satellite appears in Transmit from and Receive at like a ground station.

Transmitting from a satellite sends a downlink: the signal leaves the satellite's antenna at its position in orbit, and moves with it.
Receiving at a satellite hears the uplink: the signals that reach the satellite's antenna in orbit.

On a satellite the tracking control is called Doppler-correct toward and takes one of your ground stations. A transmission from the satellite is then corrected to arrive on frequency at that station, and reception at the satellite is corrected for signals from it.

Other people hear a public satellite by receiving at their own ground station while it passes, or receive at it directly as a Public station.

Deorbit

A deorbit is a real retrograde burn, not a deletion.

In the satellite's details, press Deorbit…. The app works out where and when the satellite will reenter and shows the fall on the globe.
The confirmation shows how long until it reenters and over which place, the size of the burn and how many sessions will end.
Press Fire deorbit burn to confirm, or Cancel. The burn can't be undone.

After the burn the satellite is marked Deorbiting. It keeps working on its falling orbit: sessions run, passes are listed and others can still use it. The Reentry panel counts down to the moment it reaches 80 km and burns up. At that moment its sessions end and it disappears from the map.