Transponder group delay

The radio science measurements at the company Astrium in Toulouse in january
2003 should determine among other things the group delay in the ranging 
channel of the FM-transponders 1 and 2.
Due to insufficent time and technical problems only the delay in X-band for
transponder 1 could measured. In the following an estimate for the 
difference between delays in S-and X-Band shall be made.

1. For the receiving channel of the transponder the signal path ways of
S- and X-Band beginning with the first intermediate frequency (ZF 
intermediate frequency IF) are identical.There is only a difference in the
area in front of this first ZF. The components in the S-Band and X-Band 
pathway are the same and consist of a LNA (Low Noise Amplifier), an image
rejection filter (bandpass) and a conversion transducer.

The group delay for each amplifier in a LNA + substrate (circuits on the 
boards) are smaller than 1 ns just like the group delay for the conversion 
transducer and filter are smaller than 1 ns (estimate due to a measurement).
These values are for the S- and X-Band range nearly identical. The absolute
group delay originating from the components before the first ZF amounts 
therefore to few nanoseconds and is the same for X- and S-Band.

A resulting difference is created essentially through different path ways
to the ZF-Combiner. Here no values are available, therefor in first oder 
zero as a value is assumed. Bigger path differences can occur within the
RFDU (30 cm = 1 ns * sqrt(epsilon_r_path way))

2. There are no group delays in the digital range.

3. In the transmitting area the ranging signal is directly modulated onto
the carrier signal and is amplified subsequently in the power amplifier.
For X-Band the ranging signal is modulated to 440 f_1, in the following the
frequency is double dampened and is the fed to the TWTA. It is estimated 
that in this area the difference in group delay is not greater than 5 ns
(X-Band group delay is larger). For path way differences in the RFDU see 1.

The group delay measurements within the ranging channel in X-Band show for
1,1 MHz a great deviation from the usual run.

The variance in the single stages is shown in the following figure (see pdf
first graph). 

For 1,1 MHz and from 2/3 * (Delay1MHZ+Delay 1,15 MHz) = 2.722 micro seconds.
(approximation with a linear equation)

For the PRARE mission the bias between S-Band and X-band delay was calculated 
from the differencec between day and night measurements in the ionosphere.