a flexible, transceiver-based rf communications system for small ...

A FLEXIBLE, TRANSCEIVER-BASED RF COMMUNICATIONS SYSTEM FOR SMALL SATELLITES Christopher C. DeBoy and Matthew J. Reinhart The Johns Hopkins University Applied Physics Laboratory Laurel, MD 20723 USA 1.

ABSTRACT

This paper describes the development and performance of a highly integrated RF transceiver system. The system is card-based, allowing for ready incorporation into an integrated spacecraft electronics module, and scalable, enabling easy adaptation to meet varying mission requirements. A description of the transceiver architecture is presented, highlighting particular advantages in cost and size savings. The use of commercial off-the-shelf parts and a recently developed, non-coherent, two-way doppler tracking technique is also reported, as are performance results on transceivers built for two NASA missions that will use these cards. 2.

INTRODUCTION

The current emphasis on lower cost satellites has fueled interest in smaller, more integrated telecommunications equipment. At the Applied Physics Laboratory, a highly scalable, highly integrated transceiver architecture has been developed to address this issue. This card-based architecture is modular in application, with separate elements comprising the RF, analog baseband, and digital circuitry for both the receiver and transmitter. This modularity gives the cards a great deal of flexibility in meeting the varying requirements that each new mission presents. The card-based approach itself allows for direct insertion of the bulk of the RF communications system into an integrated spacecraft electronics module, reducing the overall harness. Additional cost and size savings are realized with the use of commercial, off-the-shelf parts (including plastic encapsulated parts) and with the use of a non-coherent, two-way Doppler tracking system that obviates the need for a transponder-based system and offers attractive tracking alternatives for non-GPS tracked missions, e.g., highly elliptical orbiters. This work has resulted in a set of S-Band transceiver cards for TIMED, a spacecraft built for NASA by APL that will study the upper atmosphere from low-Earth orbit, due for launch in 2001. The scalable architecture has also allowed for a natural extension of the initial S-Band design to create a set of X-Band transceiver cards for the APL-built CONTOUR spacecraft, one of NASA’s Discovery missions, to be launched in 2002. 3.

THE TIMED RF TRANSCEIVER

NASA’s TIMED spacecraft makes use of an integrated electronics module (IEM) to centralize many core spacecraft functions, thereby reducing mass and overall costs. A significant contribution to the net mass and cost savings has been the inclusion of the

RF transceiver system within the IEM. The transceiver operates at S-Band, with uplink and downlink functions provided via two separate RF cards. The uplink card tracks the uplink carrier and demodulates 2 kbps command data. (The uplink is the NASA standard, a 16 kHz subcarrier with the data that is phase modulated onto the carrier.) The 15cm x 22cm card itself (see Figure 1) is comprised of three separate circuit boards sandwiched about an aluminum heat sink: an RF board for downconverting the S-Band input to an intermediate frequency, an analog board supplying the carrier tracking, automatic gain control, and local oscillator generation functions, and a digital board to demodulate the commands. The IEM approach has allowed TIMED to incorporate more functionality into the uplink card than is traditionally applied. A real-time Critical Command Decoder is included on the card and can execute any relay command on the spacecraft. With the uplink card constantly powered, this adds an extra element of capability during anomalous or unpredicted spacecraft operating modes when other IEM electronics may have been switched off. The S-Band downlink card is similar to the uplink card in structure (see also Figure 1): an RF and analog board on one side of an aluminum backbone, a digital board on the other side. The card outputs a CCSDS-compatible downlink signal at S-Band, and supports data rates to 4 Mbps (using DQPSK). The downlink card also incorporates an on-board data framer and Reed-Solomon encoder, functions that in less integrated spacecraft system designs might be found in separate units.

Figure 1: TIMED Uplink Card, left, and Downlink Card, right (RF/Analog Side) Components for both cards are completely surface-mount, with commercial, off-theshelf components used where appropriate and where reliability criteria were met. Several field programmable gate arrays were used in the digital boards to greatly reduce parts count. As is evident in Figure 1, surface mount shields were applied around sensitive areas on both cards, principally higher frequency functions, to lessen any electromagnetic susceptibility effects.

4.

NON-COHERENT NAVIGATION

A highly accurate, non-coherent, two-way doppler tracking system adds unique capability to the transceiver system. Developed recently at APL [1][2], this technique obviates the need for coherency between the uplink’s carrier tracking oscillator and the downlink carrier, reducing the complexity of the RF system hardware and providing a tracking capability for missions that do not or can not use GPS (for example, highly elliptical orbiters.) In this technique, the uplink carrier signal is received and compared with the receiver’s on-board reference oscillator. This operation results in a set of phase comparison counts that is placed in telemetry and relayed to the ground. There, the ground station continues to track the spacecraft as if it were tracking a coherent transponder. Naturally, errors will result due to the lack of coherency between uplink and downlink and the drift of the spacecraft’s reference oscillator. Both of these errors are corrected with the telemetered phase comparison counts. The additional telemetry data requirement varies from approximately 0.5 bps to 24 bps. Tracking accuracy to 0.1 mm/s (velocity error) has been achieved. 5.

CURRENT WORK - CONTOUR

The scalable nature of the card system allowed for a natural extension of the initial SBand design to create a set of X-Band transceiver cards for the APL-built CONTOUR spacecraft. Similar in size and structure to the TIMED cards, the CONTOUR cards provide a communications capability at X-Band, achieving a lower receiver acquisition threshold while still retaining the inherent advantages that the transceiver-based approach offers. The uplink card on CONTOUR has been modified to meet mission requirements to provide a low carrier acquisition threshold (to –155 dBm) in addition to an X-Band track capability. The three board arrangement (RF downconverter, analog board, and digital board mated to an aluminum heat sink) has been retained. The downlink card has added a x4 multiplier to generate an X-Band output, and the output power has been adjusted to meet the mission specification. Photos of the engineering model cards are shown in Figure 2.

RF/Analog Side

Digital Side

RF/Analog Side

Digital Side

Figure 2: CONTOUR Uplink Card, left, and Downlink Card, right

6.

CONCLUSIONS

A flexible, card-based transceiver system has been presented. The unique architecture offers a number of advantages to small satellite missions, including mass and cost savings, modularity and scalability, and a highly accurate, non-coherent Doppler tracking capability. APL has completed and delivered two sets of S-Band transceiver cards to NASA’s TIMED spacecraft. Flight unit testing on CONTOUR’s X-Band cards is proceeding apace, with delivery to the spacecraft expected in the fall of 2001. 7. 1. 2.

REFERENCES J. R. Jensen and R. S. Bokulic, “Highly Accurate, Noncoherent Technique for Spacecraft Doppler Tracking”, IEEE Trans. Aerospace and Electronic Systems, 35, 963-973 (1999) J. R. Jensen and R. S. Bokulic, “Experimental Verification of Non-Coherent Doppler Tracking at the Deep-Space Network”, IEEE Trans. Aerospace and Electronic Systems, 36, 1401-1405 (2000)