TerrasarX and TandemX are currently extremely close together. To track both it hitherto took between 15 and 30 seconds to stop tracking one satellite and start tracking the next satellite.

Since the total time available for tracking a pass of the satellites at Hx is typically only 120-150 seconds, the time taken to swap between satellites is significant and previously the observer would observe one satellite for half the pass and then swap to the second satellite for the remainder of the pass.

To overcome this restriction, we have devised a scheme which tracks both satellites as a single satellite pass, automatically changing from one satellite to the next every 15 seconds. This change is seemless and gives continuous data from one satellite to the other.

Our current prediction system forms a number of polynomials (the number depending on pass duration) for geocentric x, y and z which in real time are converted to topocentric Azimuth, Elevation and Range. We thus do normal predictions for both satellites and then combine the sets of polynomials for TerrasarX and TandemX into a single file changing polynomials every 15 seconds.

Plotted above is the raw residual plot that the observer will have seen in real-time. The residuals for both look to be very close because they are plotted against their individual predicted ranges. When it comes to the reductions the range residuals for the second satellite are a long way off the data for the first satellite when plotted against the predictions for the first satellite, alowing unambiguous extraction of pre-processed observations.