Phase 3D GPS ReceiverPhase 3D GPS ReceiverThis page and all referenced images are Copyright 1996 by AMSAT.
All text is by N3EUA, who accepts sole responsibility for any inaccuracies.
The Phase 3D GPS Experiment is primarily intended to demonstrate our ability to determine the orbital position, and three-axis attitude of the spacecraft by analysis of the radio signals emanating from the constellation of GPS satellites. There are several secondary experiments planned, the most interesting of which is our intent to map the (unpublished) antenna patterns of the GPS satellites outside their orbits, since Phase 3D's orbit will take us high above the GPS constellation at apogee.
The unusual requirements placed on a GPS receiver in the orbital environment dictate the need for a receiver somewhat different from the typical GPS receivers available for terrestrial use. We set out to build a custom receiver for the P3D GPS project, based on the GPS front-end chipset from GEC Plessey and a processor board based on the AMD 29200 embedded RISC processor. The CPU portion of this custom receiver is essentially ready to fly, with flight and flight backup units through most of their test suite.
However, the RF board housing all of the Plessey parts has not been completed, and so we are now engaged in executing a contingency plan.
The receivers will interface to the Rudak communications processor using 9600 baud async RS-422 interfaces. Software will be written for Rudak to support a command and telemetry interface to the Trimble receivers.
The motivation for two receivers centers on the fact that we have two very different operating environments at apogee and perigee. In one case, we're below the GPS constellation seeing relatively "normal" signal levels at our antennas. In the other case, we are far outside the GPS constellation and perhaps only receiving signals from GPS satellites around the far side of the Earth. Thus, we have very different antenna and preamp needs for the two cases, and it's easier to use two Trimble units than try to build switching circuitry in front of them. It also gives us a bit of redundancy.
Two styles of patch antennas for the two operating conditions have been donated by Ball Bros. More detail on these antennas will be posted eventually.
We are designing custom preamps to meet our needs.
A custom power switch has been designed to give the IHU ultimate control over the availability of power for the GPS receivers, but to also give Rudak the ability to cycle power on the receivers to better control their operation, and provide for the ability to do a 'reset' without needing IHU command team involvement.
More information will be posted as time allows.
A December 1995 Update on the Now-Defunct Receiver Project.