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Rocket Onboard GPS

ONBOARD GPS AVIONICS SYSTEMS

The GPS, or Global Positioning System consists of a fleet of 24 orbiting satellites that transmit precise position (latitude, longitude, altitude) and time information. A small receiver unit (generally hand held) enables the GPS signal to be received just about anywhere on the surface of the Earth (assuming the receiver is able to receive a signal from the satellites), and as long as the receiver can obtain a signal from multiple satellites, then the position of the receiver on the surface of the Earth can be determined.

With the decrease in cost of GPS chipsets, the GPS has started to infiltrate the rocketry avionics scene for both use in rocketry recovery (indicating the position of a rocket in conjunction with a data transmitter), and for providing 3-dimensional logging of a vehicle's flight trajectory, which can then be downloaded to a computer. At the most advanced level, a GPS unit onboard a rocket can be coupled to a telemetry transmitter, and with suitable software on the ground, the output can be displayed on a moving map, to indicate exactly where the rocket is in the air, to enable it to be located far more accurately.

A GPS unit is restricted to working at upper limits of 999 knots and 60,000 feet. Either parameter can be exceeded, however, all data above that value will be recorded at that maximum value. The reason for this is because of restrictions on the export of high technology which could be used for nefarious purposes. The restriction is set by COCOM.

ONBOARD GPS - THE SIMPLEST METHOD

The simplest way to fly a GPS onboard a rocket, is to take a handheld unit, and fit it into a rocket airframe. For the smaller GPS units such as the Garmin eTrex series, this enables a GPS unit to be fitted into an airframe as small as 54mm diameter. A number of units such as this have been flown successfully in High Power Rockets, and have returned 3-dimensional data.

This, in fact, is the way I have flown onboard GPS systems, with my handheld Garmin GPS eTrex unit launched onboard one of my rockets. The GPS unit did maintain lock during flight.

ONBOARD GPS - THE CLEVER METHOD

The clever solution to flying a GPS onboard a rocket (in terms of ease of operation and data recovered), is to buy a GPS module for the R-DAS Flight Computer. The R-DAS GPS module interfaces to the R-DAS to enable full data logging of the GPS data, and when connected to the R-DAS telemetry module (which operates at a frequency of 433 MHz), enables the GPS data to be telemetered back to the ground receiving station in real time, on either a laptop PC running the Windows operating system, or a Palm held Windows CE device. The software also allows the GPS data to be graphed in 3 dimensions.

ONBOARD GPS - THE HARD METHOD

The hard solution to flying an onboard GPS is to roll your own - i.e. scratch build a GPS unit from off-the shelf GPS modules, and develop a scratch built interface to any external devices (i.e. a scratch built telemetry system), and scratch built software to decode the telemetry signal into GPS data. This generally requires a reasonable level of electronics knowledge, and decent soldering skills, not to mention programming skills.

GPS Modules:

- GPS Modules are available from companies such as Micro-Blox, Sigem and Garmin. Maplin sell the Garmin GPS Module (Order code: VX20W) and Sigem GPS Modules (Order codes: ZQ87U and ZQ88V). The mico-blox module is used in the AED R-DAS GPS Module.

Telemetry Modules:

- Suitable telemetry modules in the U.K. would operate at a frequency of 433 MHz (an MPT 1340 compliant frequency). Examples of suitable modules would be those available from companies such as Radiometrix (The TX2 transmitter), and Quasar. Ideally, an encoder needs to be connected to the telemetry transmitter and a decoder connected to the telemetry receiver - this reduces errors in the transmission. However, it is possible to send an unencoded data stream via these telemetry transmitters - it just means the quality of the data cannot neccesarily be relied upon.

Software:

Windows - For Windows software, a program called GPSU (GPS Utility) enables the data from GPS units to be downloaded to a PC. GPSU is available both as a free download, or as a higher specification version.

Linux - A Perl software module is available for connection to GPS units. This enables software to be written to call pre-written routines which allow the GPS unit to be queried, and for data to be downloaded.

Windows CE - Various software is available for use with GPS systems and Windows CE. The one I have used is GPSEasyCE, which seems to work fairly well as a simple moving map with a Garmin eTrex connected via a serial cable.