wiki/ avionics/ av3-gps

Introduction

Any UAV needs to GPS to aid it's inertial sensors, and our rockets are no exception.

Requirements

  1. Small form factor, low power
  2. Ultra-fast relock
  3. As high dynamics as possible
  4. SBAS compatible
  5. Pseudorange data available (so we can do our own position/aiding/etc calculations)
  6. If possible: USB interface

Future plans

We plan to spin GPS subsystem of the AV3 avionics system in three phases:

  1. Off-the-shelf GPS receiver (2012 - 2013?)
  2. Add GPL-GPS software and/or FPGA receiver (2013?)
  3. Transition to GPL-GPS only receiver (2014?)

COTS GPS receiver board: The Hemisphere Crescent OEM SX-2

We're using the "Hemisphere GPS" Crescent OEM SX-2 GPS board. Some interesting PDFs:

Notes: The binary pseudo-range data is only available at 1 Hz. See binary data files, one short (1 minute), one long (~5 minutes). The following log files contain binary data from the hemisphere GPS. Refer to the integrators manual for the data structure of messages BIN96 and BIN99.

We did a capture of binary messages here: hemisphere-capture-2012-08-23 10-30.bin (106K)

Messages

We set the baudrate to 115200 for the obvious reasons, and then set everything to 1 Hz. We think we want the following messages, where "see" means we want most of this information in a dashboard display, and "want" means we want the data from this message for future usage.

MSG # See? Want? Description & Why
Bin 1 Y Y Stuff That We Want (NumOfSats, Latitude, Longitude, Height, VNorth, VEast, Vup, NavMode)
Bin 2 Y Y Navigation solution information Which satellites are used in the nav solution
Bin 80 N Y WAAS message
Bin 93 N Y WAAS ephemeris
Bin 94 N Y ionospheric and UTC conversion parameters
Bin 95 N Y Satellite ephemeris data for all channels
Bin 96 N Y Phase and code (pseudorange) data
Bin 97 N N? Processor utilization
Bin 98 N Y Satellite almanacs
Bin 99 Y Y Individual GPS satellite tracking data (SNR for each satellite)

GPS Subsystem Information

As always, there's a lot of "glue" surrounding any particular subsystem. Parts of the subsystem are:

Carrier Board

The Power enable pin in the FTDI chip is tied to the enable line of the SPS so we can selectively shut down power to the GPS subsystem.

There is a hole and a notch near the antenna connector to feed a loop of coax between the GPS board and the LNA.

Questions