On May 1, 2000 the intentional degradation of GPS signals (called SA) was terminated. With this change a GPS without a DGPS unit attached became almost as accurate as a GPS with a DGPS attached. What follows is the result of 2 separate 24 hour tests, one with DGPS and one without.
The tests were both done using a Motorola M12 Oncore GPS (sw. version 1.3) outputting Motorola Oncore-format Binary data which was logged to an attached NetBSD computer. The GPS antenna was a roof-mounted Trimble/Lowe antenna with 15 meters (50 ft.) of RG58A/U connecting the antenna to the GPS.
For the second test a Garmin GBR-21 DGPS radio tuned to the Point Blunt, CA Coast Guard station was used to provide the DGPS corrections. The Coast Guard station was located a mere 53 km (33 miles) away and provided the faster (thus lower delay) 200 baud corrections.
samples: 86448 (24.0 hrs)
ave lat: 37.55 degrees
ave lon: -121.94 degrees
ave alt: 19.068 meters
50.00% confidence: 4.2 meters
68.27% confidence: 5.7 meters
95.45% confidence: 12.2 meters
99.73% confidence: 23.6 meters
Alt
50.00% confidence: 7.6 meters
68.27% confidence: 10.8 meters
95.45% confidence: 23.9 meters
99.73% confidence: 39.5 meters
What this means is that a user taking a single non-DGPS correction can expect the reading to fall within 4.2 meters of the correct position 50% of the time. Similarly the reading should fall within 12.2 meters of the correct position 95% of the time.
samples: None yet
ave lat:
ave lon:
ave alt:
50.00% confidence:
68.27% confidence:
95.45% confidence:
99.73% confidence:
Alt
50.00% confidence:
68.27% confidence:
95.45% confidence:
99.73% confidence:
The M12 Oncore ver-1.3 under test doesn't appear to work correctly with Coast Guard DGPS correction messages (Type-9/3). This test will have to wait for a firmware upgrade.
The exact position of the rooftop antenna is not currently known. The position of the rooftop antenna is derived as the average of the 24 hour position. If there are any biases in the system, or if the GPS were to have an error in calculating its position correctly then some undetected errors could sneak in.
The height plotted here is what Motorola refers to as the "GPS height".
The above statistics may be gathered with a home grown unix program I call salog. Feel free to grab the above distribution and run your own statistics. It should compile up with little difficulty under netbsd, freebsd, openbsd and linux.
You will have to put the Oncore into NMEA mode to use this program.
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wolfgang.rupprecht+web@gmail.com
(Wolfgang S. Rupprecht)
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