Description: The elements of the X-Set shown in Figure 1 were an X Receiver, a Data Processor with a software program loader (Philips cassette), a Power Supply designed for both 400Hz and 28V DC power, a Battery Backup, a Control Display that had, in addition, the ability to load program patches thru the keyboard, a Pilot Steering Display and a Preamplifier. The equipment shown in Figure 1 dissipated over 700 watts and weighed 700 lbs. The data processor alone weighed 150 lbs and dissipated 500 watts. This was characteristic of the parts available in the 1974-5 timeframe. X Receiver The X Receiver was a 4 Satillite/Psuedolite continuously tracking receiver designed to meet the full performance specifications including jamming and had the ability to track to high levels of jamming when aided. The X Receiver's functional design had the ability to use two antennas and to use either L1 or L2 for each channel. This mode of operation had 4 channels tracking carrier signals and the fifth channel time-sharing code tracking. The mode changed to code tracking with high jamming. The X Receiver was substantially a digital design and samples were transferred to the Process Controller (PC), part of the receiver, at a 1 KHz rate where all of the signal processing was done in software. Magnavox built the PC from available computer chips (ALU, RAM, ROM and I/O) and micro coded to emulate the Hewlett Packard 2100 minicomputer family.
Data Processor and Software The Data Processor was a rugedized version or the HP 2100 Minicomputer( HP 21Mx) with semiconductor memory. It had 64,000 words (17bits) of memory and a throughput of less than 20,000 floating-point ops. /sec. Not nearly as good as even microprocessors of today but enough to do the job.
The software, except for interrupt processing and receiver processing, was done under a sub-contact to Intermetrics of Cambridge Massachusetts, who in addition developed a Structured Higher Order Language (MELTRAN). MELTRAN translated into ANSI Fortran IV and allowed for maximum readability, ease of understanding, testing and maintenance. It is believed to be one of the first uses of a Structured HOL in military software development.
The navigation solution was updated at a fixed rate of 10Hz. This task was the only fixed rate; all other tasks were done on a priority basis with the Kalman Filter of the lowest priority and used whatever residual through put remained. The 8 state unaided Kalman Filter update rate was once a second while the 11 state aided filter updated once every 8 seconds. Both used pseudo range and carrier cycles over the update interval as measurements for the Kalman Filter.
The Weapons Delivery task had a fast loop and a slow loop and computed the release time to 10 msecs. Although the weapons officer “pickled” weapons release, it was the X-Set that actually released the weapon.
Test Results The Phase 1 validation tests at Yuma Proving Grounds (YPG) during the period between March of 1977 and June of 1979, as well as some of the military demonstrations outside of YPG, validated the viability of GPS use in the military as well as the civilian sectors. It was the military demonstrations that did the most to convince the DOD that GPS could move into Phase II IOT&E. The Magnavox Sets, and particularly the X, did what they were supposed to do. The Magnavox set's performance in those demos is depicted in Figure 2. The Weapons Delivery tests dropped over 1000 MK82 bombs during this period.