The Trimpack Global Positioning System (GPS) navigation receiver is a self-contained navigation aid. It utilizes the Coarse Acquisition (CA) code and the L1 frequency (1575.42 MHz) of the Navstar Space Vehicle (SV) constellation to obtain three dimensional position and velocity information. The system features the ability to track up to seven satellites and the ability to determine position without any position initialization.

Acquisition. The time required for the Trimpack to acquire satellites and calculate position and velocity is dependent on a number of factors. However, if the Trimpack has a current almanac, current ephemeris, and has not been moved a great distance (<1000 kilometres) since its last fix, acquisition and position/velocity solution should occur within 1.0 to 1.5 minutes from power-up.

In normal operation, the Trimpack will select the best available signals from four satellites to provide a 3-D fix. If only three satellites are available, the calculated positions are limited to horizontal, 2-D, co-ordinates and the altitude label flashes. In this mode, the accuracy of the fix depends on an accurate estimate of the altitude, whether from the previous position or operator entry.

The Trimpack provides for an automatic transition between 2-D and 3-D modes. While in 2-D mode, the position display flashes every 30 minutes and/or every 1 mile in position displacement from the last fix to indicate that you should enter a new altitude. When 3-D fixes resume, the Trimpack disregards the entered value and displays the GPS calculated altitude.

Signal Interruption. Obstructions causing interruptions in the satellite signals or natural conditions which degrade signal strength can lengthen acquisition times. Generally, for the first minute of the interrupt, the Trimpack continues to search for satellites at the last frequencies to which it was locked. If the signal is regained during this minute, reacquisition will be almost immediate, provided platform velocity has not changed by more than 90 knots. If the velocity has changed by more than 90 knots the Doppler frequency has shifted. Then, the Trimpack shall finish its 1 minute search of the previous frequencies before expanding the search to reacquire. The search time will then depend on the amount of velocity change, but is still usually within 15 seconds. However, if the signal is blocked for more than 1 minute, the expanding frequency search will have already begun. In this case reacquisition may require up to 6 minutes or longer.

NOTE

If the signal is interrupted for more than 1 minute, reacquisition may be faster if the system is turned off then on (power-up cycle).

Dynamic Capability. For proper Trimpack operation, vehicle velocity is limited to less than 300 m/s (–600 knots) and should not exceed 2 g’s (19.5 m/s/s). The GPS system is designed to operate over a temperature range of –30 to +65 degrees Celsius.