SA Watch allows you to monitor and analyze the wandering that your GPS receiver appears to do when it is at a fixed location. Originally, this wandering was largely the result of SA - Selective Availability - DOD- speak for the deliberate degradation of the accuracy of your GPS signal. When SA was eliminated as a source of position error, we got our first good look at the remaining errors. SA Watch lets you see these graphically and statistically.
Look for SA Watch Instructions on Joe Mehaffey and Jack Yeazel's GPS Information Website.
What's New?
Version 3.80 adds support for the TSS POS/MV and similar receivers that send a $INGGA sentence with "bad" checksum. See the new GPS/GPSIgnoreChecksumErrors menu command.
Version 3.77 fixes some "runtime error 6"
bugs.
Version 3.75 adds an option to not dither the
display of logged data and a few other, minor enhancements.
Version 3.74 adds support for the Garmin "eSeries"
receivers, an expanded "Snapshot" of the Line Graph and
EPE-scaled position fix circles.
Version 3.61 corrects weighted mean of
altitude when reviewing logging files.
Version 3.60 fixes a long standing bug with very
large logging files.
Version 3.50 brings SA Watch into the post-SA era.
Position Errors computed to 10cm.
Map Scales down to 25mm/pixel.
Line Graph Y-axis scale down to 1m.
Latest Sample and Mean circles resized.
Filter on Position Error down to 10cm.
Filter on EPE down to 1 meter.
Optional auto-incrementing image save filename.
Serial port data rate to 19,200 bps.
Serial port RTS handshaking supported.
SA Watch Help updated.
The SA Watch screen image below shows data collected at the NGS benchmark, S159, located near Slaughterhouse bridge, Aspen, CO. Note that the mean of all samples collected was just 2.2 meters from the USGS surveyed location of the benchmark and 99% of all samples were within 5.6 meters of the benchmark!
In previous updates, Version 3.41 introduces a new single file installation. Version 3.40 fixes a long standing bug with very large logging files. Version 3.32 corrects two minor bugs. Version 3.30 adds Internet DGPS. Evaluate accuracy of your GPS receiver without investing in a . SA Watch uses an Internet connection to receive DGPS corrections originating from the Point Blunt, CA, USCG DGPS beacon to provide your GPS receiver with a continuous stream of correction data. Check out the SA Watch image below, captured from Boulder, CO. Of more than 1,800 samples, 99% were within 4 meters of the reference point! This feature is of greatest value for observers within 2,000 km of San Francisco, CA, but useful insights can be gained from most of North America. Thanks to Wolfgang Rupprecht for the data stream.
Version 3.28 adds Journaling of raw NMEA data with the File/JournalRawNMEAData menu command The complete NMEA data stream is displayed, in real-time, in a separate viewing window and, optionally, journaled to a file for subsequent processing. These journaled files can be read by SA Watch using the File/OpenNMEASampleFile menu command. Both of these features are available only to registered users. Version 3.27 resolves problems for users who switch their Windows Regional Settings. Version 3.26 fixed a nasty bug that resulted in a "Runtime Error 5" in a new installation. Version 3.25 added Map Dithering for DGPS users. With DGPS, the stability and apparent accuracy of the mapped fixes can far exceed the reported precision of some GPS receivers. Map Dithering, when enabled in the Option menu, dithers the mapped dots to better reveal the actual density of mapped fixes. Thanks go to Dr. Tom Clark for this inspiration. Other features of version 3.2x of SA Watch include support for multiple, named, reference points; direct entry of reference point coordinates, selectable HDOP break points and elevation display. Version 3.2x offers full support of the TAPR DGPS Reference Station in both GPS receiver and DGPS reference station modes, including support of the high accuracy clock signal for system time-keeping.
SA Watch provides insight into the effectiveness of the various strategies your GPS receiver uses to disguise or minimize the effects of SA. Using SA Watch you can learn to distinguish between the artifacts of your GPS receiver's firmware and limitations of the GPS system.
The effective use of GPS in the field is strongly influenced by your understanding of the subtle interaction between time, your GPS receiver, its antenna and every object that intrudes on the antenna's view of the sky. Exploring GPS using SA Watch from a fixed location will help you develop that understanding and permit you to make the most of your field use of GPS.
SA Watch can help you answer such questions as:
How accurate is my GPS receiver?
Are position errors symmetrical?
How long does it take for averaging to significantly improve a
fix?
How does antenna position affect HDOP?
How does HDOP effect the accuracy of a fix?
How many SVs does it take to obtain stable fixes?
How many SVs are in the sky over time?
What is my GPS Horizon?
How does my HDOP change as the trees leaf-out?
How does weather affect HDOP?
Is the specified 30 meter accuracy, 95% of the time, realistic?
Is the Garmin "Estimated Position Error" (EPE)
realistic?
SA Watch can capture a log of data spanning many days and multiple logging sessions. It can then export that data in a format that is ready for advanced statistical analysis with programs like MS Excel.
SA Watch is a 32-bit application for Windows9x/NT/2000 systems only.
Download SAWatch380Setup.exe (2.2 MB). The SA Watch installation file, last updated: 13 April, 2002. This installation file will install itself into the directory of your choice.
Note: SA Watch requires the latest Microsoft run-time support files. If you experience problems installing SA Watch such as "missing files" errors it may be that you do not have these latest files. In that case, download and install the self-extracting file, Vbrun60sp5.exe, from the Microsoft Visual Basic Run-Time Files site, then reboot Windows and reinstall SA Watch.
Registration of SA Watch is just US$20 and may be charged to your VISA/Mastercard account. Registration remains valid for updates.
SA Watch processes standard NMEA sentences available from many GPS receivers. Supported receivers include the Eagle Explorer, Etak GPS-1, Garmin 12, 12XL, II+, 20, 30, 35 and 40, Magellan 3000, 4000XL, Meridian and DLX10; Lowrance GMS, Motorola Oncore, Motorola Oncore with TAPR DGPSIB, Trimble Scoutmaster and Tripmate. At the minimum, SA Watch requires only the $GPGGA NMEA sentence - use the GPS/GPS Receiver Type command and select "Generic $GPGGA".
Support for other GPS receivers is added with nearly every version. To this end, SA Watch incorporates the capability to collect a data stream sample of your GPS receiver's NMEA output. By collecting this sample and emailing it to the author you can help bring full support for your receiver to SA Watch.
SA Watch is being developed and tested using a Garmin 12XL and Motorola Oncore with TAPR DGPSIB.
On April 20th SA was apparently off from 0400 to 2000 UTC and significantly reduced from 0100 to 0400 UTC. Sa Watch (version 1.0) captured this dramatic event. View SA Watch screen clips and Excel charts of an analysis of the data exported from SA Watch.
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Send email to the author, Stan Huntting, KW7KW (ex-KF0IA).