I have a problem concerning GPS units. I have been studying on a project for a weather balloon which will go about 30 - 40km up. Now I do know that some GPS units have a limit on how high they can receive/show you the location whether it is a limitation made by the military to stop random production of cruise missiles and others which just don't work above a certain height. This is a problem as my weather balloon does go to a height which could be considered as a "GPS fail zone". At the moment I am using a 50 Channel D2523T Helical GPS Receiver - http://www.sparkfun.com/products/9566 . It’s a great product but I am worried with problems with height etc. Could anyone give me some ideas on GPS units or give me advice....Any help is greatly appreciated....P.S - The unit has to have the standerd GGA protocol.
GPS Units are typically limited to controlled airspace (60kfeet, 18km), and 1000 knots. These are commonly known as the COCOM limits. For anything outside of that, you will probably have to go for a higher-end GPS unit and possibly some additional paperwork.
I believe that the uBlox module that is on that board implements the COCOM limits correctly. In the uBlox G5010 datasheet, it gives the operational limits as 972 knots and 50km.
The DoD specification (I believe) says that a GPS unit should not operate above 60kfeet and 1000 or knots, but most of the manufacturers don't actually implement it this way. The commonly cited list of high-altitude capable GPS receivers is here.
Some other companies produce GPS units to spec: most notably Inventek.
The International Traffic in Arms Regulation law (AKA ITAR) defines as a munition, in section § 121.16 -- Missile Technology Control Regime Annex:
...The following items constitute all items on the Missile Technology Control Regime Annex which are covered by the U.S. Munitions List:
Item 11-Category II
Avionics equipment, "technology" and components as follows; designed or modified for use in the systems in Item 1, and specially designed software therefor:
(c) Global Positioning System (GPS) or similar satellite receivers;
(1) Capable of providing navigation information under the following operational conditions:
(i) At speeds in excess of 515 m/sec (1,000 nautical miles/hours); and
(ii) At altitudes in excess of 18 km (60,000 feet), (see § 121.1, Category XV...)
Category XV - Spacecraft Systems and Associated Equipment part C similarly defines as a munition:
(c) Global Positioning System (GPS) receiving equipment specifically designed, modified or configured for military use; or GPS receiving equipment with any of the following characteristics:
(2) Designed for producing navigation results above 60,000 feet altitude and at 1,000 knots velocity or greater;
Your low-speed balloon should (in theory) be fine, because it's never going to exceed 1,000 knots. However, many GPS receivers implement conformance by operating neither above 60,000 ft nor at speeds greater than 1000 knots.
The datasheet for your module states, on the technical specifications page, under "dynamic conditions" that it's not defined to operate at greater than 1,000 knots. I don't see any maximum for altitude. Therefore, you need to call or email them.
One thing that I do see, however, is the GGA specification on page 8, which formats the MSL Altitude as %5.1f - You won't get the right result above 99,999.9 feet. I'm not sure if this means it overflows, or what happens. Again, you need to contact the manufacturer.
The rules known as CoCom are discussed here: http://en.m.wikipedia.org/wiki/CoCom
Australia is a signatory.
There is a nice discussion of GPS units suitable for High Altitude Balloons (HAB) at not surprisingly the UK High Altitude Society (UKHAS) here: http://ukhas.org.uk/guides:gps_modules
By the way the Geohelix GPS cited is no longer produced as the company has gone out of business.