The system is also provided with 10" of Z-axis travel for changing the probe to antenna spacing, and a rotator for use with single polarization probes. The spacecraft antenna is mounted on a cart with nearly 5' of vertical motion to position the antenna at the correct height with respect to the scanner.
OPTICAL STRUCTURE MONITOR - Because the system is located in a seismically active area in California, a basic design objective was to allow for and compensate for some movement of the structure due to ground settling and small earthquake effects. An optical structure monitoring system was designed and implemented to track probe X and Y linear positions, and probe X,Y, and Z errors. The system uses HP laser interferometers for the linear position feedback, and a custom designed NSI optical subsystem for measuring the probe position errors. The probe Z position error is derived using a spinning plane laser tracked by a sensor at the probe. A major advantage of this optical structure monitor concept is that it reduces the design requirements on the scanner and can yield a much more cost effective design than traditional mechanical design methods.
RF PROBES - Four dual-polarization probes were implemented to cover the bands of interest. Because the test antennas were large and the far-field coverage requirements were quite narrow, high gain probes were desirable(1). The Ku-band probe was dual-linear, and the lower frequency probes were dual-CP. Comsat was selected to design and fabricate the probes, and NIST gain and pattern calibrations were performed on each probe. Figure 4 shows one of the probe assemblies.
RF EQUIPMENT - The RF subsystem is designed around a Hewlett Packard 8530A receiver. The Hewlett Packard equipment was selected because of its high performance and excellent reliability. The multiple frequency switching requirements and cable lengths dictated selection of the dual-source, external mixer system configuration for the HP-8530. The H50 mixer option was chosen which allows operation of the system up to 50 GHz without the need to purchase additional source or receiving equipment. Figure 5 is a block diagram of the RF equipment implemented on the range.
COMPUTER EQUIPMENT - The system runs from a single Compaq 486/50 computer, with a second identical computer provided for additional data processing capability. The systems each have 36 Mbytes of RAM and a 500 Mbyte hard disk drive with tape backup. Computer interface cards include the IEEE-488 interface to the RF equipment, and a Digital Signal Processor (DSP) for interfacing with the optical subsystem.
The optical module reads the various sensors and derives the probe X,Y and Z position errors. These errors are then used to provide either corrections during data processing, or to actually correct the probe position real-time while scanning. For instance, small movements of the X and Z axes while scanning the Y axis can be used to keep the probe moving in a straight line.
The synchronization module required the PC to take direct control of the HP-8360 sources. Both units are fed with a frequency list at the beginning of each forward or reverse scan. The sources can then be triggered from the PC while scanning with a TTL trigger line. Since the list of one unit is offset from the other by 20 MHz, the HP-8530A receiver remains phase locked and in its fast data mode during the entire measurement sequence. The PC simply triggers the receiver at the appropriate times and reads the data from its buffer. The frequency lists are scanned in reverse order on the reverse pass to insure the near-field points at a given frequency are spatially aligned with the forward pass.
The NSI software also has a built-in expert system and an automated test sequencer. The expert system aids the operator in designing the near-field test and insuring the test parameters selected will yield acceptable results. The automated test sequencer allows complex test scenarios to be set up by an experienced range technician, and run by novice operators with a minimum of keystrokes. These enhancements help increase the productivity of the system, and are described in further detail in another paper(2).
|Scan Area||22' x 22'||Scanner to fit in existing Loral chamber|
|Test configuration||Antenna pointing up w.r.t. gravity||Scanner elevated 25 ft above floor.|
|Frequency range||General requirement 2.5-40 GHz Optimized for 5 bands 2.5-15 GHz||Wide frequency range - used HP H-50 external mixers|
|Multiplexed data while continuously scanning||2-polariz. by 4-Beams by 10-Freqs||Fast, accurate source and receiver triggering - NSI DSP synchronizer|
|Gain measurement accuracy||0.25 dB||Probe gain calibrated at NIST|
|Cross-polarization accuracy||0.75 dB at -27 dB||Low cross-pol probe - Comsat laboratories|
|Pointing accuracy||0.02||Accurate Z-plane|
Figure 1 - System Design Requirements
Figure 2 - NSI Scanner Y-axis Bridge Above SS/L Antenna
Figure 3 - NSI Scanner Testing SS/L Antenna
Figure 4 - Dual-Polarization Probe Assembly
Figure 5 - SS/L RF Sub-system Block Diagram