2016 Sep 28
Wednesday, Day 272
Satellite predictions for observing and tracking
On screen tracking in real time
Orbital elements (requires Registration)
Partial copy of Space Track information (Registration not required)
Software to track satellites on your PC
Doppler Curves in Satellite Tracking
Measuring the Doppler Shift is a satellite tracking technique for determining the distance between the satellite and the receiver at the time of closest approach as well as the time itself. As a satellite approaches, the frequency appears raised relative to the actual transmission frequency. As it goes away, the frequency appears to be lowered. At the time of closest approach, the transmitted and received frequencies are usually the same.
A measurement of frequency against time produces a Doppler curve. As the satellite passes, the received frequency appears to fall but not in a constant manner. The rate of change starts off slow, is greatest at the time of closest approach and then tails off towards the end of the transit. This is because we are measuring the rate of change in the component of a satellite's velocity along a line joining the satellite to the receiver. The rate of change is greatest at the time of closest approach and, if measured, can be used to determine passing distance.
Below is a Doppler curve produced from the GFO pass around 1600 GMT on 2000 December 8. This is an early attempt at producing a curve - from before the days of plentiful availability of the hardware and software that allowed frequency data to be harvested and plotted directly. It was made by reading-off the frequency from a radio receiver at intervals and then entering the results into a spreadsheet in order to create the plot.
In the space of seven minutes between 1558 and 1605, the received frequency dropped from 400.041 MHz to 400.024 MHz. The closest approach computes as 873 kilometres against an orbit height of 790 kilometers, so the pass was nearly overhead. Analysis of a prediction based on Mike McCants' element set produced 840 kilometres but given that the margin of error in the Doppler calculation is 50 kilometres, then they agree.
With the advent of Software Derived Radios, Doppler measurement became somewhat easier. First, a standard display immediately shows the shape of the Doppler curve. Below is a classic curve from an satellite in LEO, in this case - Progress M-63. The software used is Spectrum Lab. It is much better for this job than the software the came with the SDR receiver but unfortunately, although it is still being developed and updated, it does not run on Windows versions later than XP.
In this case, the precise transmission frequency is known so the time of closest approach can be read directly from the time scale.
Plotting a Doppler Curve
The output from a Software defined radio can be collected as a set of frequency readings when using a piece of software like Spectrum Lab. This allows it to be analysed more closely than is possible from a simple screen dump, and there are different ways of manipulating it. This plot was created in a spreadsheet using frequency measurements sent out directly from the receiver/software combination.
Measuring the Time of Closest Approach
Closest approach is at the time the frequency is changing most rapidly as a result of the Doppler Effect. Because the software-based receiving system outputs a frequency measurement in digital format, then it is possible to calculate and plot the rate of change in frequency during a pass of a satellite.
This plot uses the same data that produced the Doppler curve in the diagram above. The time of closest approach came at the minimum point on the curve where the rate of change in frequency has its greatest magnitude. This particular data set had some gaps due to fading of the signal but it still allowed measurement to ±5 seconds in time.
A Different Doppler Curve
The actual transmission frequencies of some satellites can be controlled from the ground. The method used involves the satellite locking onto a transmitter on the ground and then transmitting at a frequency that is some fixed ratio of the frequency that it receives. For satellites using the USAF SGLS (Satellite-Ground Link System) the ratio is 256/205. SGLS Channel 9 for example has and uplink at a very precise, and tightly controlled, 1795.752 MHz. The downlink is 2242.500 MHz.
When tracking a satellite that is transmitting in this mode, the frequency often seems to change at a different rate from would be expected if the satellite was simply generating its own signal. The reason is that the signal reaching the ground from the satellite carries the combined Doppler effects of the relative movement between ground station and satellite, and between satellite and tracking station.
It makes calculating the time of closest approach more complicated because the location of the ground station needs to be known in order to subtract its Doppler Effect from the received signal so that a usable Doppler curve remains.
The diagram here relates to one of the STSS Demonstrator satellites. It shows two different Doppler curves for a pass by the Zarya tracking station at Lincoln in the UK. The red line is the actual received signal, the blue line is the same set of data with the uplink Doppler shift removed. Given that the ground station's location in the UK was known, the calculation was simple. Were the location not known then it would have been a little more complex and some trial and error might have to be involved.
When left to its own devices, the satellite transmitter was sitting at 2242.516 MHz and that is the frequency that would have beeen measured by any tracking station at the time of closest approach. Once 'locked' it was totally controlled by the Doppler-shifted frequency received from the ground station. The transmitting ground station itself would see the frequency as 2242.500 MHz at closest approach.
This is a real time view of signals from the STSS Demonstrator satellites as seen from Lincoln in the UK. The first curve shows the effect of the transmission being locked to the frequency of an uplink from the USAF Satellite Control Network's UK ground station. It combines both Doppler shifts. The second curve illustrates what happens when the uplink lock is broken, the received frequency becomes a conventional Doppler curve. The curve to the right is the start of a 'non-locked' Doppler shifted signal from the second STSS Demonstrator satellite travelling a few minutes behind.
In this example, the Doppler curve was quite normal for a 'non-locked' transmission. Suddenly, the received frequency hopped downward by 38 kHz. Then the status quo was restored a little over a minute later when it jumped upwards by 41 kHz. It was as if the satellite's receiver had responded to, and locked onto, an uplink for a few seconds but the large frequency jump indicated that it was not the usual UK ground station near to the Zarya location.
The observed change indicated that the particpating ground station was a long way from the UK and to the west. At the time the event occurred, the only land masses to the west of Europe visible to the satellite were the extreme NE coast of Canada, Greenland, and Iceland. Of these, the most likely location for the uplink transmission was Greenland.
Modelling the effect of an uplink from the western coast of Greenland produced very similar results to those observed, suggesting the source of the uplink was the USAF Satellite Control Network ground station in that country.
What may have happened was that Greenland opened up a communications session (possibly with the other STSS Demonstrator satellite) and this satellite picked up the 'lock' command. As it moved towards the horizon as seen from Greenland, it lost the uplink and the lock was released. The Doppler curve then returned to its earlier form.
Copyright © Robert Christy, all rights reserved
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