Check system timing accuracy (using astrometry of GNSS satellites)

Overview

Pointing the telescope

Exposure Length

Example Images

Repeat the Process

Position Measurement

Results

Periodically re-check

Non-timing errors
 

Overview

This is an outline of how to use several online software tools made available by Bill Gray (see MPML message #32538) allowing the overall timing accuracy of a telescope system to be determined by measuring the position of Global Navigation Satellite Systems (GNSS) such as the US GPS, European Galileo and Russian GLONASS satellites.

The orbits of these communications satellites are very well known, from where they were soon after launch, some dating back to the early 1990's to where they will be up to about 24 hours into the future and to an accuracy of just a few centimetres.

As these satellites are generally moving at apparent speeds of 2,000 - 2,400 arc seconds per minute, or 30 - 40 arc seconds per second, they can be used as a very sensitive gauge of the absolute timing accuracy of an imaging system, with all the delays and errors introduced by the camera shutter, imaging software, operating system, timing source and even observatory topocentric coordinates (see non-timing errors) combining to affect the final measurement.

The positional accuracy in RA & Dec can be expected to be of the order of 1 arc-second or better but a timing error of 1 second in the clock will manifest itself as a very obvious 30-40 arc-second discrepancy with the ephemeris, so providing a very sensitive measure of timing accuracy.

Configure capture software

Of course the recorded time may not be accurate but at least it will be saved to two decimal places. However, the GNSS tools described here will allow you to determine how accurate the times really are.
 

Pointing the telescope

• Choose a target from a list of GNSS satellites visible at your site (gps_find)
• Generate an ephemeris for the selected satellite (gps_eph)

Exposure length

At Great Shefford, with a 2.2"/pixel scale and a 3-pixel radius aperture in Astrometrica, a 0.3 second exposure allows the resulting 10-12 arc-second trails to be easily measured.

With very short exposures there may not be enough reference stars recorded to achieve a good astrometric measurement, so check that there are catalogue stars distributed across the field and that the target has reference stars on all sides (in Astrometrica tick the Reference stars option within the Images/Select Markings... menu option).
 

Example images

Here is an example showing a typical short trail from a 0.3 second exposure:

The trail is short enough for the measurement annulus in Astrometrica to determine a good centroid:

Repeat the process

Depending on how quickly your system can take consecutive images you may need to reposition the telescope a number of times to get enough images to determine a statistically relevant result. These GNSS satellites pass through the 18 arc-minute field of view of the telescope at Great Shefford Observatory in about 30 seconds and with a gap of 1 - 3 seconds between exposures about 10 measurable images can be obtained from one telescope pointing.

It is suggested to obtain at least 10 positions, preferably more and from a number of different fields of view and in different directions in the sky, using more than one GNSS satellite.
 

Position Measurement

The next step is to measure astrometric positions of the target satellite. The date and time needs to have enough precision in the astrometry output file so that the accuracy of the method can be fully utilised. In Astrometrica this means making sure that in the CCD tab of the Program settings the "Time in File Header" precision is set to 0.1 seconds or better.

The resulting astrometry, either in the MPC's 80 column format or in the new ADES format can then be copy/pasted into the link below to work out how far off the measurements for each observation are in along-track seconds of time and cross-track arc-seconds:

• Check GNSS astrometry (gps_ast)

Results

Here is a sample of the resulting output from gps_ast, with most of the detail lines removed for clarity:

Avg cross-track : -0.018644 +/- 0.338065"
Avg along-track (timing): -0.0050475 +/- 0.0382892 seconds
Negative along-track errors mean your clock was 'ahead' of the actual time;
i.e., the times reported in the astrometry are later than the positions
of the GPS satellites would indicate.

This value was then configured in Maxim/DL (in the Camera Control, Expose tab, Options, Measure Shutter Latency... option) so that every subsequent image taken has the 0.07 second system latency automatically applied to the time recorded in the FITS header. In the gps_ast output above, this adjustment has already been applied, reducing the overall timing latency to -0.005 ± 0.038 seconds.

Combining measurements made on three different nights and plotting timing errors horizontally in seconds of time and cross-track errors vertically in arc-seconds, the random distribution of measurements can be seen here:

Periodically re-check

Non-timing errors

(Satellites are identified in the figure with the 3-character codes from gps_find)

After the observatory coordinates were corrected, both sets of points are now properly centred around 0 arc seconds on the cross track (Y) axis: