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STSS Demonstrator Mission


Tyneside, UK
2018 Jul 17
Tuesday, Day 198

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STSS Demonstrator home page
S-band Frequency Variations

The two STSS Demonstrator satellites each use a channel in the USAF Space-Ground Link System. The SV1 satellite uses SGLS Channel 10 where the centre frequency is 2247.5 MHz and SV2 uses Channel 15, centred on 2272.5 MHz. Although the centre frequency of each channel is precisely defined, neither satellite transmits exactly at that frequency.

Actual Transmission Frequencies

When there is sufficient power available from the satellite solar cell/chemical battery storage system, the transmission frequencies of SV1 and SV2 appear to be 2247.499 MHz and 2272.514 MHz respectively. Allowing for the fact that the radio receiver used is the AR-8600 (a hobbyist receiver) from AoR, it is subject to small errors in tuning. The frequencies may well be 2247.500 MHz (the channel centre) and 2272.515 MHz (15 kHz higher than the channel centre).

Frequency Changes

The transmitted frequency varies in two ways. First is when the satellite's receiver locks onto an uplink transmission from a ground station. The received frequency is stepped up by the ratio 256/205 to become the transmission frequency. Both uplink and downlink are affected by Doppler shift. There is an example of this on the "Ground Station Frequency Lock" page available from the left hand menu.

Secondly, when free-running and not locked to a ground station, the transmission frequencies of both satellites move around. There is plot of observed changes below. The red, upper trace is SV2 and the blue trace is SV1. The left hand scale is a measure of the offset from the centre frequency of the channnel.

Since launch, SV1 has been the steadier of the two showing a slow rise over the first few days in orbit, before a sudden drop but then it settled down to near constancy. There is small variation and it seems to come in steps with several days of consistent measurements passing before the next up/down step.

SV2's changes on the other hand are much more dramatic. The initial rise lasted much longer than that of its companion and then stepping was much larger, initially also giving the impression of being more frequent.

STSS Demo Frequency Variations

The following paragraphs include notes on frequency changes. The final section on this page is an extract from the "Log of Events" where the entry is particularly relevant to the transmisssion status.

Cause of the Changes

There are some pointers from observations of other satellites at S-band. Some geosynchronous satellites can be seen to exhibit a distinct upward change in frequency on entering the Earth's shadow with a slow return to normal after going back into sunlight. This seems to tell us that a higher frequency is an indication of power demand exceeding availability. The up and down steps in the STSS Demonstrator frequencies may indicate variations in the onboard power balance as the result of equipment being switched on and off.

If this is a correct conclusion then the plot suggests SV2 to have been the 'busier' of the pair during the first few weeks in orbit. Given that the two significant orbital adjustments were both undertaken by SV2 then this may be true. A pessimistic view on SV2 is that there might have been an problem with the onboard power supply after it first reached orbit but that it has now been resolved.

Settling Down

SV2 made two orbit adjustments in the first few weeks. The first was 2009 October 7 when the separation rate was reduced. At the same time, SV2's frequency ceased its initial rise and SV1 exhibited a jump from 2247.497 MHz to 2247.500 MHz. Afterwards, both satellites continued to exhibit small changes with SV2 being the most active.

November 12 saw SV2 undertake its second notable orbit adjustment. Immediately afterwards, both satellites seemed to have 'settled down' in frequency terms with only small variations evident up to December 1 when SV2 exhibited a sudden upward shift of a few kHz.

Both satellites seem to go quiescent over the Christmas/New Year period, possibly as staffing was reduced on the ground for the holiday. SV2 enjoyed a much longer 'break' than SV1. Over the period, neither satellite exhibited significant variations in frequency.
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