Functional Requirements

The ground GPS hardware provides data that can be used by the GPS software for differential corrections.

Interface

Power: Standard 110 AC power outlet Data output: 9600 bps, RS232 serial stream.

Connectors (2): power and data.

The power connection is a Standard AC plug.

The pin out for the serial connection is listed in table 3.6.3.1 below.

 

Pinout for GPS serial connector
9 pin Serial
1	NC	6	NC
2	Tx	7	NC
3	Rx	8	NC
4	NC	9	NC
5	Ground

Design

The major ground station components are the receiver, the antenna, AC/DC adapter, PC, and the ground station board. See Jay Helmericks’s Masters Thesis for a more complete description of the GPS system. The ground station board serves as the necessary interface between the GPS, external power and the computer. The board has a five volt regulator so that any dc supply from 6.5v to 14v can be used. The power connector is a round power jack so that standard AC/DC converters can be used. The board can then be powered from any AC outlet. Currently a 9 volt 300 mA AC/DC converter powers the prototype board. The center post of the plug is ground and the outer shield is V+. The other major part of the board is the logic level to RS232 serial converter. This is a Motorola chip that takes the 0 to 5 volt serial stream from the GPS and converts it to RS232. This provides the necessary interface for a computer to collect the data. There are two LED’s on the board that monitor +5v and the receiver pulse per second. Ths allows the visual monitoring of main power and receiver functioning. The receiver mounts to the ground station board and is electrically connected through a 20 pin 2mm female header. The receiver is physically mounted using four standoffs and bolts holding it securely in place. The antenna is a Micropulse 12700. This is a robust antenna that can be securely mounted using four bolts. The antenna has a TNC connection. RG-316 cable is used to run from the antenna to the MCX connection on the receiver. The antenna needs to be in a place with a good view of the sky. It is desired that the ground antenna sees all the satellites that the flight GPS sees that are above 5 degrees. The antenna must be placed at a known surveyed location, or allowed to collect data for several hours to fix its location. Once the location is fixed, the antenna can be removed and replaced, as long as it is put back in the same location.

Assembly Notes

When the board is constructed the antenna hole must be cut out to the edge of the board. A hole is drilled during fabrication where the antenna plug goes in. This hole needs to be extended to the edge of the board for the antenna cable to follow. To make sure that the receiver board is level and not bending the connecting pins a few washers will need to be added to the standoffs. The height of the pins doesn’t correspond to a standard mounting height.

Component Testing

Power up the board and verify 5v Check LED for receiver operation Check serial output and verify correct receiver operation

 

more info:
SRP-4 design Document Appendix A