Callisto has developed a number of bespoke simulators for testing ground equipment prior to a satellite launch.
The METEOSAT series satellite are spin stabilised and are equipped with Electrically De-spun Antennas, for the communication links to the ground stations. The switching elements in the Electrically De-spun Antennas cause phase and amplitude ripples at the satellite spin rate to be introduced on the communication links. In order to test the effect on the ground reception equipment Callisto was asked to design and develop simulators to produce parasitic AM and PM ripple on data-modulated RF signals. RF voltage variable attenuators and voltage variable phase shifters were used, driven by a programmable waveform generator. The simulator includes software running on a PC which allow the User to control the parameters of the waveform generator; frequency, amplitude, phase and waveform type. In addition complex waveform generation from file sources can be used.
Many satellite missions start with an In Orbit Test (IOT) prior to the start of operations. Dedicated IOT equipment is installed in ground stations to undertake these tests. However, how can the ground IOT equipment be verified and the test procedures validated before satellite launch? In the case of the MSG satellite, Callisto was asked to design a transponder simulator for this purpose. The unit was specified with an uplink at 2.1 GHz and a downlink at 1.7 GHz. Inside the simulator there was down-conversion of the input 2.1 GHz signal to an IF of 110 MHz IF, followed by an up-conversion from 110 MHz to 1.7 GHz. The unit included filters to simulate the transponder characteristics, step attenuators to simulate the link losses and amplifiers to simulate non-linear behaviour.
Verification that ground station equipment can operate with Doppler effects due to satellite motion is important, particular for LEO missions when Doppler shifts and rates of frequency change can be high. Callisto has designed and developed equipment which can accurately simulate the change in frequency on a downlink RF signal as if coming from a satellite pass.
The simulator, which works for either an S band downlink or an X band downlink, calculates the Doppler profile for a pass based on the operator selecting a few orbital parameters for the mission to be simulated (orbital height, inclination, maximum elevation angle). The calculated Doppler profile is then used to drive a synthesised Local Oscillator, which produces a frequency shift on test signal connected to the IF input of the ground station receivers.
The Doppler profile is updated in real-time to simulate a satellite pass. Care has been taken in the design to ensure that the frequency changes are phase continuous, as will be the case with a real satellite signal.
It is sometimes important to test a ground segment telemetry chain with real satellite telemetry data, rather than simulated data. In the case when a satellite suitcase is not available, then a solution is to record telemetry from the satellite at the satellite integration facility, then playback the data at the ground station. Callisto has developed a TM Recorder based on a PC. The unit can record either a video signal (modulated sub-carrier for example) or a PCM serial data stream. The length of data recording is limited only by the amount of free space on the PC hard drive.