As outlined in Rockwell's Validation Phase (Phase I) proposal, the Global Positioning System (GPS) space segment has nine subsystems: navigation; telemetry, tracking, and command; attitude and velocity control; reaction control; electrical power; orbit injection; structural; thermal control; and radiation hardening. This document is a summary of the nine subsystems. The designs are "baseline," with options available for later development. Figures 1 and 2 show the proposed spacecraft.
Please note that the source for this material is dated 1 April 1974. This was Rockwell's proposal for a GPS test system of four satellites. The system would have a lifetime of five years, and would provide operations and user information for the design of the operational satellites. The space vehicles would be in two or three half-sidereal-day orbits, inclined 63°. The orbital ascending nodes would be separated by 120°. Orbit phasing would be controlled so that all the vehicles would appear over a test area each day, allowing three dimensional navigation within that area. Phase II of the program would provide two-dimensional worldwide coverage, and Phase III would provide a continuous worldwide three-dimensional navigation system.
Figure 1. Satellite Hardware 3-View.
Figure 2. Satellite Hardware Locations.
Navigation Subsystem
The navigation subsystem provides simultaneous down link of two L-band carriers, L1 and L2. The L1 signal (1575MHz) is biphase modulated with two codes: a protected 10.23MHz pseudo random noise (PRN) code (P code), and a clear acquisition 1.023MHz PRN code (C/A code). The L2 signal (1230MHz) is biphase modulated with the P code only. "Non-secure" users use the C/A code for ranging, while "secure" users use the C/A code for acquisition and the P codes for precise ranging. Secure users use the two P codes to correct the satellite pseudo ranges for atmospheric errors, and the P codes do not include the corruptions intentionally inserted into the C/A code.
The navigation subsystem comprises four main assemblies: clock, navigation control, PRN signal, and antenna assemblies. Component characteristics are listed in Table 1 at the end of the section.
Clock Assembly
The clock assembly generates a 10.23MHz signal that drives the L1 and L2 frequency synthesizers and the PRN baseband generator. The clock must have long-term stability of 7´10-13 for 105sec. The frequency standard is a set of three rubidium optical resonators. The rubidium standards are lightweight, easy to execute, and meet the stability requirements. The three oscillator signals are sent to a distribution amplifier unit. The amplifier unit amplifies the signal from one selected frequency standard, and sends the signal to the L1/L2 carrier synthesizers and the PRN baseband generator. A third output signal is used for group delay calibration, an indication of errors accumulated between the frequency standard and the antenna diplexer (Section 2.3.6).
The clock assembly has normal and backup frequency standard modes. In the normal mode, a voltage-controlled crystal oscillator is locked to the resonance of a rubidium cell. If a rubidium loop fails, the backup mode has the crystal oscillator operate in open loop. The output frequency may be digitally controlled by controlling the magnetic field around the rubidium optical resonator cell, and by controlling the crystal oscillator voltage. The smallest clock adjustment is four parts in 1012.
A cesium clock standard was planned for the operational phase (Phase III). The cesium standard degrades less than the rubidium standard in the space environment.
Navigation Control Assembly
The navigation control assembly contains two navigation decoders, a command data distributor, and a secondary power unit. The main component is the decoder unit. The decoder unit receives demodulated signals from the telemetry, tracking, and command (TT&C) subsystem and decodes the signals. It provides for processor and memory updates, frequency standard adjustments, and it routes commands.
PRN Signal Assembly
The PRN signal assembly generates the coded PRN signals and sends them to the antenna array. The Phase I vehicle has three operational modes: normal, high-power, and L1. Under the normal mode, the L1 transmission has 10.2W for the P signal and 20.4W for the C/A signal, and the L2 signal is 6W. High-power mode uses excess beginning-of-life power capacity to transmit a 38.8W C/A signal on L1, while L2 is unchanged. In L1 mode, the space vehicle does not transmit the L2 signal, and the L1 signal may be normal or high-power. The maximum transmission slant range, defined as 5° above a ground observer's horizon, is 25,345km. The PRN signal includes a 50-bit-per-second (bps) data stream containing satellite ephemeris coefficients, clock corrections, telemetry and status identification words, and variable data.
Eight functional units make up the PRN signal assembly: navigation telemetry encoder; PRN baseband/data memory/processor; L1/L2 carrier synthesizer; L1 modulator/intermediate power amplifier (MOD/IPA); L2 band MOD/IPA; L1 band high power amplifier (HPA); L2 band HPA; and the antenna diplexer. This assembly gets regulated power (±12V and ±5V) from two identical power supplies, each with a backup.
Navigation Telemetry Encoder
The navigation telemetry encoder provides a 7-bit data word to the PRN Processor for insertion with the status identification word of the navigation signal data frame. This word indicates the navigation control state.
PRN Baseband/Data Memory/Processor
The PRN Baseband/Data Memory/Processor unit generates the P and C/A PRN codes, stores uploaded messages, reformats and generates navigation data, and formats selected subsystem telemetry for insertion into the data frame. The unit contains three basebands, processors, and telemetry encoders, which are individually selectable and cross-strapped for redundancy.
The processor subassembly receives data from the data decoder of the Navigation Control Assembly, and from the Navigation Telemetry Encoder. The processor sends the data to the baseband subassembly through processor interface circuits. (The processor is designed to handle 36 lines 8-discrete input, and 28 lines 4-discrete output.) The processor interface reads clock counter data into the processor, sends system data from the processor to the baseband P-code generator, and relays commanded resets.
Each baseband subassembly contains a P-code PRN generator, a C/A-code PRN generator, and a clock. The subassembly has three output drivers: two for the L1 signal (P and C/A codes) and one for the L2 signal (P code only). The data stream is added to the PRN stream in the baseband. The output signals go to the L1 and L2 modulators.
L1/L2 Carrier Synthesizer
The carrier synthesizer receives a 10.23MHz frequency from the frequency standard distribution amplifier, and generates two output frequencies: L1 at 1575MHz and L2 at 1230MHz. The unit performs the frequency multiplication using step recovery diodes, which are simple, low-noise, and efficient. The output frequencies are sent to the MOD/IPA units.
L1 and L2 Modulator/Intermediate Power Amplifiers
The L1 MOD/IPA produces a quadri-phase shift keyed (QPSK) modulated signal at 1575MHz, and amplifies the signal to drive the L1 high power amplifier. The QPSK signal is composed of bi-phase shift keyed (BPSK) modulated P- and C/A-channel signals. The P and C/A signals come from the PRN Baseband/Data Memory/Processor unit.
The L2 MOD/IPA is patterned after the L1 unit. It processes the P-channel only, producing a BPSK modulated signal at 1230MHz.
L1 and L2 Band High Power Amplifiers
The L1 HPA amplifies the 1575MHz QPSK modulated signal input from the MOD/IPA. Output power is fed to the antenna diplexer. The main amplifier is redundant, and an incremental power amplifier module provides extra power for the high power mode. Normal mode radio frequency (RF) power is 37.2W, while high power output is 58.3W.
The L2 HPA is nearly identical to the L1 HPA, but produces 8.5W of RF power.
Antenna Diplexer
The diplexer combines the L1 and L2 signals from the high power amplifiers into one antenna feed. The unit uses three-pole filters (coaxial cavity resonators) for each signal, and a combining tee network.
Antenna Assembly
The PRN antenna consists of twelve helical elements on the forward bulkhead, parallel to the Z-axis (Figure 1). The twelve elements produce a shaped beam. The shaped beam provides constant illumination on Earth across the field of view, and wastes the least energy beyond Earth. Each element is an aluminum wire around a fiberglass cylinder (Figure 3). Two of the TT&C antennas are also located on the forward bulkhead, on the same mast (Figure 3). The TT&C antennas are positioned for minimum degradation of the navigation transmissions.
Sunday, 01-Aug-2004 00:36:47 CDT
CSR/TSGC Team Web
