Magnavox GPS reference station
A GPS Reference station typically has two functions. It logs data for postprocessing and it provides a realtime RTCM104 stream for postprocessing in the field.
Firewall note: For a few months I had public access to my Magnavox enabled via a web page, then one day Comcast cut off my account without any notice. ("Hello, how may we give you excellent service today?") To get the account turned back on I had to firewall off my home web site. Eventually I will work out another method for demonstrating the GPS receiver that will not irritate Comcast. I will probably periodically upload data to a public server.
History
This story started when I picked up a GPS base station at a garage sale, almost unused. It's a Magnavox MX-9112 DGPS receiver. It also came with a set of Pacific Crest Radio Modems and some nifty antennas, which I no longer have.
- MX-9012R - DGPS Reference Station
- MX-9112 - operates as either DGPS Navigator or Reference Station
- MX-9212 - DGPS Navigator (Leica stickers on the side)
- MX-9400N Leica branded DGPS Navigator
What makes my MX-9112 extra interesting (to me anyway) is that it came in a case that is unlike anything in the documentation. I wonder if it is an early production model. Maybe the ones in the docs are MX-9212's.
I bought a MX-9212 from c-Com on eBay and it's in a box that's about 10% of the size of this older unit. I've used it as a time base for an server rack several times.
I bought the later model Leica MX-9400N on eBay too. It's in a sturdy finned aluminum box.
There is also an MX-9400R reference station that comes in the same box.
Why would I want a GPS base station?
Well, now I can keep track of exactly where I live. That's what people assume when I tell them the antenna is for GPS. Also I will use it to learn more about GPS.
There are several projects that I want to work on.
- I'd like to set up a GPS-based NTP server.
- I'd like to track the GPS satellites. I now have both perl and python cgi scripts available to read data; you can try the python gps cgi. (Well-- you can if you are behind my firewall! See note at top of page.)
- If I collect data with rrdtool then I'd include weather data so that I could correlate conditions and SNR. See Doing something about the weather
- I want to collect correction files so that I can try doing postprocessing on data collected from a rover. To do this I will probably need to watch what gets collected in the DOS program and write some python code to do the same thing in Windows.
- Later on I'd like to set up a data connection and try realtime RTCM-104 correction with my rover.
- The system came with a DOS floppy disk; I was able to run the program under MSDOS in a VMWare machine. I really don't want to run VMWare all the time though, so I am looking at writing a Python version so that I can run it under Linux as my first project. (And learn a little more Python in the process.)
The basic software can do several things. The MX-92xx can be used as a conventional GPS receiver, providing current location and velocity. It can be used as a reference station, issuing a stream of differential data to allow both real-time (RTCM-104) and postprocessing of GPS data. To be a reference station, the station has to know its own location. This can be determined via surveying, or you can run the station in a 'learn mode' where it will average out many thousands of readings and store the position.
The system also came with pretty good documentation.
It talks NMEA on the serial port, so first thing I need to do is learn to speak its NMEA dialect.
So here we go: Talking to the Magnavox
Operational models
I am now writing a daemon to provide an interface between the blue box and the rest of the software that I will write.
When I decide to make the thing real-time, the daemon will have to listen to the 1 PPS signal from the box and respond accordingly; for example, it has to handle NTP requests.
I don't need to store data until I am ready to try post-processing.
My immediate desire is to be able to request the most recent data from the box. In my initial tests, I wrote a CGI program that listened to the box until it had enough data to display, this takes about 10 seconds.
So the first cut of the daemon will do this:
- Listen to the continuous NMEA data stream and keep an in-memory copy of the status updated. Pretty much this has already been written in read-gps.cgi.
- Listen for requests on a port, and upon receiving one, send the current dataset in XML format. This is basically a web service.
The next pass will be to accept commands from a client and pass them back to the box.
The story continues with these notes on the Magnavox Daemon
The original material here has no copyright notices on it anywhere and I presume that Leica does not care if I post copies here. If you are Leica and you care, say so and I will take the material down.
Power connection
On the MX-9212, it's a coaxial connector with a 2.5 center pin (center positive) and it runs on 11-32 volts. On the reference station it has a Hirose connector. I'd like to know the specs so I could buy another one.
What the LEDs mean
At power on, all three LEDs blink at the same time.
RED - Power on
YELLOW - passed self test, attempting to acquire satellites (red off)
GREEN - flashing - Acquiring satellites but not navigating
GREEN - solid - Navigating (or computing corrections) - other LEDs off
Documentation
Still to be scanned: The Operator's Manual,
The Magnavox DGPS 12 Channel Installation and Service manual covers how to install the GPS and its antenna and how to wire it up. File:Magnavox DGPS 12 channel Installation and Service.pdf
The Magnavox DGPS 12 Channel Technical Reference manual covers the protocols, this is especially of interest if you want to understand its dialect of NMEA-0183 (it emits custom sentences that communicate reference station data.) File:Magnavox DGPS 12 channel Technical Reference.pdf
This is an earlier doc, just a few critical pages from the manual. File:Magnavox Installation and Service Manual selected pages.pdf
Scanned and converted to PDF at 300dpi using a Brother scanner with automatic document feed and Adobe Acrobat Pro 9.0 along with some Javascript downloaded from the Internet to support page reversing and collation.
See also How to scan documents
Software
This is an image of the software that came on a floppy disk. It runs under DOS, I can provide you with a VirtualBox appliance image that includes the Magnavox software and FreeDOS if you don't happen to have an MSDOS machine around. Just ask.
You need this program to change settings on the receiver. Also it does the DGPS function of logging data to files.
It is fully described in the Operator's manual.
