Some aircraft data are read via RS-232. For this, we are using the standard /dev/ttySxx ports and drivers. The aircraft data are in a 9600 baud stream occurring once per second and the GPS produces a position message twice per second. We use our GPS message to drive a simple Blt plot of the latitude versus longitude, so we can track the progress of the flight.
The RS-232 data are actually captured by the rgc program, since the RT-Linux modules can't make use of the native Linux drivers and we didn't need to rewrite drivers that were working perfectly. Once the data are read, they are copied to the shared memory area above 61MB where any of the programs can access it. Normally, it is accessed by another invocation of rgc and read.
Accurate aircraft attitude, heading and track angle data are critically important to the SRA in real time. The pitch-and-roll attitude of the aircraft is taken from the on-board Inertial Navigation Units (INU), using Synchro-to-digital converters—one for each parameter. These are read by the RT-Linux module during each scan line. The heading and track information is presently provided via RS-232 from the on-board aircraft data system, which has a direct interface to the INU's digital data stream.
The SRA radar data are written to disk files by the rgc program. The aircraft data are captured by a separate program and written to a separate disk file. This data is normally captured for the entire duration of the flight, providing a complete flight record in a single file. The carrier-phase GPS data are captured continuously from 45 minutes before the flight until 45 minutes after the flight. The pre- and post-mission data are necessary to resolve the aircraft position to the centimeter level.
Before any Linux development was carried out, we felt it necessary to write some DOS code to work with the Gage digitizer board. Turbo-C version 5.0 was required to compile and use the Gage-supplied library. Once we were successful in getting a Gage example program to work on DOS, we worked with Gage engineers to communicate directly with the digitizer using a normal user-mode program. The main trick was to make the DOS program configure the digitizer and then exit without powering it down. The second trick was to boot from DOS into Linux; this turned out to be quite easy with loadlin.
We determined the PCI board settings for the digitizer by reading /proc/pci and then hard-coding various test programs with the values. We wrote various normal user-mode programs to become familiar with the digitizer. We were able to manipulate the digitizer card in every way except handling interrupts. The gdb debugger was a big help throughout the development.
A substantial part of the SRA software is actually firmware resident on various microchips.
Microchip provides, at no cost, a very complete and easy-to-use development package for their 16C65A (and other) microcontrollers. It sports a comprehensive simulator, making it possible to watch simulated execution of quite extensive programs. The only downside is the system runs only on MS Windows.
The RT-Linux extensions provide just the right features for a real-time data system such as the SRA. The extensions provide much more capability than we actually use in the SRA. We use it to start an RT task at the end of each raster scan. The task processes all the data captured during the previous scan and makes a number of calculations necessary to configure the system for the next raster.
We wrote rgc.c to be a liaison between normal user processes under Linux and the RT-Linux SRA module. Quite simply, rgc sets up a pointer to the shared memory space that the SRA RT module uses for data storage. They understand each other because they share a common .h file defining the data organization in the shared memory space. Figure 11 depicts how the various SRA real-time programs communicate with each other. rgc usually reads commands from stdin and writes to stdout. If it is invoked with certain switches, it forks and polls for RS-232 data and/or writes captured data from the shared memory to disk, all the while taking its commands from stdin. The command set is simple ASCII strings such as set thresh 24 or get roll. The Tcl/Tk programs each open a pipe to their own private rgc, then send commands and receive data back. Everything is done this way except the topographical image display. That program, creep.c (because it creeps up the screen), accesses the shared memory directly. The main reason for concentrating everything into rgc is that it generally means we need only recompile rgc, creep and the SRA module when something is added or removed in the shared memory area. In short, it makes for quicker development.
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