Satellite Communications/CPI Test



While performing cross pole isolation test with CW carrier; at what input power level this test should be done. I mean to say do we need to increase the input power so that the at receive side it reaches saturation or just before saturation...? Is there any rule or standard for power/EIRP levels at which these tests should be carried out?

Thanking you in anticipation.

Best regards,

ANSWER: Makil:

MEASAT Satellite Systems, a Malaysian satellite operator, owns and operates MEASAT and AFRICASAT satellites. The following MEASAT document describe cross-polarization isolation (CPI) tests in following document:

The CPI tests are conducted in the linear region. The test is started at 15 dB below the saturation level of the transponder. The power is increased in 1 dB increments until the transponder becomes non-linear. The power is reduced until a 1 dB reduction in the power transmitted results in a 1 dB reduction in the power received.

In other words, the test is performed near the upper limit of the linear region.

I hope that this satisfactorily addresses your questions.


Thomas E. Burke, PhD, PMP

---------- FOLLOW-UP ----------


Thank you for the quick reply. It was very helpful. After reading the document that you have provided I have two follow-up questions:

1. What if the HPA and/or antenna do not have the capability to saturate the trasnponder. Will the same rule applies in that case and we will increase the power until the saturation point of HPA...?

2. What is the formula for calculating saturation level of a single transponder? That is at what power level of a CW carrier the transponder becomes non-linear assuming HPA has enough power to do this?

Best regards,


Question 1:

I made a mistake in my original response to you. I apologize.

The goal is not to saturate the satellite transponder.

According to MEASAT Satellite Users Handbook paragraph 3.1 (Transmit Cross-Polarization Isolation Test), the test is performed at 15 dB below the saturation level of the operating transponder.

The Earth Station Under Test (ESUT) transmits an unmodulated signal at reduced power. The uplink power is increased incrementally until the Network Management Center (NMC) reports that the downlink EIRP is 15 dB below the saturation level of the operating transponder.

To verify the linearity of the system, the uplink EIRP is reduced by 1 dB; this should reduce the downlink EIRP by 1 dB. If this is the situation, one would rotate the feed by 90 degrees to transmit on the cross-pol port.

The Handbook does not specify what to do if the downlink EIRP is reduced by less than 1 dB. This would imply that the EMUT is operating in saturation. One would reduce uplink EIRP by another 1 dB. If this still failed to bring the EMUT into its linear region, one might reduce the uplink EIRP an additional 1 dB. If this did not get the EMUT into a linear (non-saturated) region,  suspect that the NMC engineer would terminate the test.

SES performs the CPI similarly, but the test is performed at 6 dB below transponder saturation level.

Question 2:

I am not aware how to calculate the saturation level a priori.

One measures a transponder’s during its unit acceptance tests and again when it’s installed on the satellite.

There is a parameter referred to as “back off” that is a measure of how far below saturation the transponder is operated. The optimum back off depends upon factors such as uplink EIRP, modulation…
Satellite transponders have a variable attenuator, commonly called Flux control Attenuator of padding.

The attenuator can be commanded from the Network Control Center to optimize communication link performance.

Thomas E. Burke, PhD, PMP

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Thomas E. Burke


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Forty years of experience with satellite communications. Have held roles as system engineer on JPL Mariner 9 program and a program manager for a number of classified communication satellite programs. Served as head of TRW / Defense Communications Division / Engineering Development Operation, a 1,400-person organization responsible for all aspects of classified communication satellite design and development.

Ph.D. Chemistry, California Institute of Technology (1969) B.A. magna cum laude Chemistry, University of Minnesota (1962)

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