GPS
Contents
ETek EB-85a
The current SWARM GPS module is the ETek EB-85a. Key features of this unit are a 5 Hz update rate, a reasonable $100 per module price, and a claimed RTCM input capability (which is non-functional as of July 9, 2007.)
Here are some important docs and support software from the Sparkfun site-
And here is a useful RTCM viewer utility-
And here is an RTCM over IP tool-
WAAS
Currently, SWARM is considering relying on WAAS as its primary source of differential corrections. Here is some info on WAAS-
What is WAAS? An alternative to RTCM based DGPS (Garmin website)
Measurement with Two Units at the Box Shop, July 14, 2007
Two EB-85a units were attached to laptops on a table in the middle of the Box Shop yard. All communication through the modules occurred through MiniGPS 1.41. The modules were set to use WAAS, and 3D DGPS indicator was active on the GUI. Data was collected with the NMEA logging feature of MiniGPS. The modules were no more than 50mm apart from one another.
A PERL script was used to extract latitude, longitude, and UTC time from the $GPGGA messages, and this data was imported into Matlab for the final analysis. The latitude and longitude were converted to meters away from a selected center position, and readings occurring at the same time as indicated by the UTC value were extracted. The hypotenuse of each difference was found, and a histogram plotted.
From this, one can observe that the maximum error is less than 5m, and is more typically less than 2m. Looking at the relative readings of the the two modules, it is apparent that a large part of the error does not appear as common mode.
GPS On The Playa
Here is the raw data that Jon took on July 4th, 2007 at the Geek Week encampment in the Black Rock Desert-
Here is the KML data processed by Matthew Deans-
Here is the analysis of about 10 minutes of stationary GPS data from the playa taken July 4, 2007-
lat_mean = 40.7668
lon_mean = -119.2279
north_meters_mean = 4.5271e+006
west_meters_mean = -1.0066e+007
north_meters_std = 0.1678
west_meters_std = 0.3681
Benchmarking Jan. 21, 2007 - Michael Prados
I brought the ET-301 to the corner of Third and 20th Streets, where there is a geodetic survey marker. Afterward, I realized the survey wasn't all that accurate, but I got 5 minutes of data. This was next to a large brick building, with several other obstructions in the area. I believe WAAS was not active, but I have some open questions about this.
mean latitude = 37.76045183888896 deg
mean longitude = 122.3884122833338 deg
north standard deviation = 0.78685413633748 m
west standard deviation = 0.29887101619448 m
Notes on GPS Antenna Stability
In order for the base station receiver to obtain the best accuracy, the choke ring antenna must be mounted on a very stable surface. Any movement of the antenna will affect the measurements gathered by the receiver. Temporary land GPS surveys often use tripods to mount the choke ring antenna. This allows the antenna to be very stable but at the same time makes it easy to remove. The base station receiver (Ashtech Z-12 can be mounted in the mothership. The GPS antenna will need to be mounted in a more stable location unless the mothership is constructed such that it does not move will sitting on the playa.
GPS Resources
Here's a link that explains dGPS (Differential GPS)
The Tucson Amateur Radio Packet APRS/GPS Site
What is WAAS? An alternative to RTCM based DGPS (Garmin website)
Navtech Supplier of GPS equipment including board modules
Boards & Modules
Byonics Carries some GPS and APRS related kits; GPS receivers, too
3-21-07 - Sparkfun has just started carrying a 5 Hz GPS module for $99 ETek EB-85A
Antaris LEA 4T GPS Unit 4 Hz Update Rate, Accepts DGPS, $180
Magellan DG-14 Board High end!
Receivers with RTCM Output (dGPS Capable)
Magellan Mobile Mapper Pro Receiver ~$3200
The Trimble 5700 A very rugged unit intended for mobile use.
The Trimble NetRS New ethernet based GPS receiver with RTCM output, runs Linux and has a web interface!
dGPS Beacon Receivers
Magellan MobileMapper dGPS Beacon Provides standard RTCM data via a serial or Bluetooth link; antenna is built in with provision for an external antenna. ~$1200
