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4. Proof of Concept System

For GPS/MET Phase I, a single LEO spacecraft/payload will be placed in orbit and operated for a limited time (nominally, 6 months). Payload performance, quantity of soundings and lifetime have been de-emphasized in favor of simplicity, schedule and budget. However, the data quality is expected to be sufficient to demonstrate the potential of the technique. Figure 4 below depicts the overall system configuration for Phase I.


Figure 4 System Block Diagram

4.1 GPS Receiver Payload

The Payload for GPS/MET Phase I consists of a modified TurboRogue GPS receiver manufactured by AOA, a dual frequency ceramic patch antenna manufactured by Ball Aerospace Systems Group, and an external Low Noise Amplifier (LNA) manufactured by MITEQ. The TurboRogue is an 8 channel, dual frequency receiver which uses a high performance RISC processor and dedicated ASIC's to implement most functions. With an LNA NF = 0.51 dB and antenna gain of 3 dB, estimated system G/T is -18 dB-K. As shown in Figure 5 below, the "omni antenna" is pointed in the anti velocity direction for the spacecraft direction of flight, providing reception of "setting occultations". The spacecraft can be yawed 180 degrees to observe "rising occultations" too. The single antenna configuration was chosen for simplicity; the only penalty being that the satellite can "see" only rising or setting occultations at a given time, not both simultaneously. The resulting antenna coverage is near optimum for receiving satellites about to be occulted while still receiving 5-7 other GPS satellites for position determination.

The receiver hardware design was modified in several ways to address the space environment. The most obvious change is a complete redesign of the aluminum case to provide optimum heat dissipation and radiation shielding. Inside, certain electronic components were upgraded (5V DC-DC Converter, SRAM, TCVCXO) and additional fail-safe circuitry was added to minimize the effect of single event upset and latch-up phenomenon. The standard keyboard, display, and flashcard related circuits were omitted.

Much of the receiver firmware has been changed to meet the special demands of the space and the mission specific occultation measurement requirements. Although designed to begin operating automatically upon power-on, the flight receiver is also highly configurable by simple ground commands. In addition, the entire code image can be loaded from the ground if necessary. The payload communicates with the spacecraft via one RS-422 port operating at 19.2 kb/s. New functionality includes: (1) 50 Hz sampling rate ("hi rate sampling") on selected satellites/carrier phase channels, (2) an occultation prediction algorithm which automatically schedules the start and stop times for high rate sampling on appropriate channels, (3) acquisition and tracking loop algorithms to accommodate LEO Doppler rates, (4) autonomous fault detection and recovery, and (5) a data compression algorithm to compress the high rate data.


Figure 5 MicroLab-1 Spacecraft

4.2 LEO Spacecraft

The host spacecraft, MicroLab-1, will be manufactured, owned and operated by OSC. This 63 kg microsat is gravity gradient stabilized, nadir pointing, with two 38" dia. sun tracking solar panels. (See Figure 5 above.) The main structure is cylindrical (41" dia. X 15" wide) with the GG boom and tip mass pointed to nadir. Attitude is controlled using three torque rods, controlled by the ACS Processor, driven by six sun, two Earth, and one magnetometer sensors. MicroLab-1 also carries a Trimble TANS Vector GPS receiver with 4 antennas for attitude determination. The spacecraft provides payload power, the required thermal environment, data storage (using a 64 Mbyte solid state recorder), and duplex S band communications with the ground. MicroLab-1 will be carrying one payload in addition to GPS/MET. The second payload is a lightning detection instrument (OTD) provided by NASA's Marshall Space Flight Center.

4.3 Launch Vehicle and Orbit

MicroLab-1 one will be one of three spacecraft launched on a Standard Pegasus from Vandenberg AFB sometime in late 1994. The other two spacecraft, also owned by OSC, are Orbcomm FM-1 & 2--so called "Little LEO" communications satellites. The planned orbit is: 775 km circular with a 70deg. inclination. First contact with the TT&C station will be approximately 9 hours after launch.

4.4 TT&C Earth Station

All communications with MicroLab-1 will take place via S band up and down links to an OSC remotely controlled Earth Station (E/S) in West Virginia. The uplink data rate will be 19.2 kb/s and down link rate will be 2 Mb/s. Both links have error detection, with manual provisions for re-transmission as necessary.

4.5 Satellite Operations Control Center

Operation of all spacecraft systems except the two payloads will be the responsibility of OSC. The spacecraft will be operated from OSC's new Spacecraft Operations Control Center (SOCC) in Dulles Virginia. The SOCC is connected to the TT&C E/S via a T1 leased line. The SOCC is connected to the GPS/MET Payload Operations Control Center (POCC) via the Internet. Payload commands originating at the POCC will be forwarded to the SOCC 30 minutes prior to a scheduled contact. Once the satellite is in view, the commands will be sent to the payload manually by the SOCC Operator on duty. Command responses will be sent back to the POCC in near real time. Science data will be transferred to the POCC within 30 minutes following the contact. Normally, there will be two scheduled contacts with the spacecraft per day to download data and upload commands.

4.6 Payload Operations Control Center

The POCC will be operated by UCAR in Boulder, Colorado. The POCC consists of a modest local area computer network with a SUN SPARC 10, Model 512 Processor at the core. Several Macintosh computers provide off-line graphical and analysis tools and X-Terminal access to the SUN. This local network is linked via the Internet to the OSC SOCC Data Server, JPL's Fiducial Data Server, and all Investigators planning to analyze the data. Primary functions of the POCC include: Incoming science data will include: (1) payload data from the SOCC, (2) standard rate IGS[21] fiducial data and GPS orbits from JPL, (3) special hi-rate fiducial data from 3 new sites installed by JPL specifically for GPS/MET, MicroLab-1 orbits processed by JPL, and operational meteorological data that will be used for validation experiments. These data will be semi-automatically retrieved and archived. Level 0 data (raw files as retrieved from the source) will be processed in near real time to produce "quick look" results in a form suitable for rapid verification that the system, and the payload flight code in particular, are functioning as expected. The processing is carried out in several steps, with intermediate results archived for investigators to use as required for a variety of experiments. The Data Products scheduled to be available from POCC are summarized below. 
                              POCC Data Product Summary

   Level     Name                  Description
     0       Raw Science TLM       Raw science data file as received from OSC.  Files
                                   contain 12 hours of data.  (approximately 12
                                   Megabytes).
     0       Ephemeris TLM         Raw MicroLab-1 attitude and ephemeris information
                                   as received from OSC.
     1       Hi-Rate Fiducial      Hz RINEX (Receiver Independent Exchange Format)
             RINEX                 data from JPL "hi-rate" fiducial sites.
     1       Normal Rate LEO       Normal rate (usually 10 sec. sample rate) RINEX
             RINEX                 files from GPS/MET Payload.  Files contain 12
                                   hours of data (approximately 2 Megabytes).
     1       High-Rate LEO RINEX   "Hi-Rate" (usually 50 Hz sample rate) RINEX files
                                   from GPS/MET Payload.  Files contain at most 1
                                   hour of data (approximately 1 Megabyte,
                                   compressed).
     1       Nav Data              Navigation solution from MicroLab-1 GPS/MET
                                   Payload.  12 hours of data (< 1 Megabyte).
     2       Double-Difference     MicroLab-1, fiducial, and POCC file information
             Meta-file             for occultation events on which data were
                                   collected.
     2       POD                   Precise orbit ephemeris for the MicroLab-1
                                   spacecraft as generated by JPL.
     2       Excess Delay          Timing, orbit, and "excess delay" information.  A
                                   separate file is produced for each occultation
                                   event.
     3       Refractivity          Geo-located refractivity profiles vs. altitude or
                                   perigee.
     4       MET                   Temperature, Pressure, Density, etc. profiles vs.
                                   altitude or perigee

4.7 Fiducial Network

As described above, ground based receiver observations are needed to carryout differential GPS processing (Double Differencing). A heterogeneous global network of approximately 30 receivers, operated by IGS, is currently used to calculate precision GPS orbits used for a wide variety of Earth science experiments. Standard rate data (30 sec.) from these sites will we downloaded from an IGS Data Center daily. In addition, precise GPS orbits produced by the IGS will be retrieved and propagated ahead on a daily basis for use in the Quick Look Data Processing. To process the hi-rate (50 Hz) phase data from the payload, hi-rate phase data is needed from at least one fiducial site in common view with the occulted satellite (and DD satellite). To meet this need, NASA/JPL has installed receivers configured to provide the required hi-rate data. These hi-rate fiducials have been installed at sites selected to provide Earth coverage representative of virtually all meteorological conditions. Figure 6 below shows the coverage from one of these sites, Fairbanks, Alaska, for a 55 day period (1/2 the MicroLab-1 precession rate).


Figure 6 DD Setting Occultations from Fairbanks (55 days, min el=15 deg., max az=60 deg.)

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