Any UAV needs to GPS to aid it's inertial sensors, and our rockets are no exception.
- Small form factor, low power
- Ultra-fast relock
- As high dynamics as possible
- SBAS compatible
- Pseudorange data available (so we can do our own position/aiding/etc calculations)
- If possible: USB interface
We plan to spin GPS subsystem of the AV3 avionics system in three phases:
- Off-the-shelf GPS receiver (2012 - 2013?)
- Add GPL-GPS software and/or FPGA receiver (2013?)
- 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)
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:
- The FC to FC breakout board connector: takes the FC USB 2.0 HS and breaks it out for the GPS subsystem
- The Hemisphere Crescent GPS receiver
- 3.3 V @ 300 mA (max)
- The GPS Source A11T 30 dB gain Low-Noise Amplifier (LNA)
- 3.0 - 16 V @ 16 mA
- "Hacked" with two 1.5475 GHz SAW filters (Abracon AFS1575.42) soldered on the PCB, one on the input, and one on the output (do we need the output one?)
- Power in to A11T is via the SMA connector from the GPS (the "OUT" SMA connector) and DC is blocked to the antenna (the "IN" SMA connector).
- RA M MCX to M SMA coax to go from GPS to LNA
- Coax from LNA to RF bulkhead
- GPS carrier board
- 0.050" SMT connector to mate with the Crescent's connector
- Cutout for the MCX cable (will it fit? Can it lock in the MCX connector?)
- RAM backup battery
- BR-1632/HFN digikey
- (Vbat or 5V) to 3.3V converter to power GPS and LNA
- Stealing from another project: TPS63001
- 6 pin 0.1" header for FTDI 3.3V serial cable
- If we can't talk USB, then an FTDI FT232R chip or equivalent
- Breakout connectors for interesting signals (1 PPS, etc).
- LEDs? More than what's on the GPS already?
- Form factor allows fitting in at top of avionics module.
- In order to put the glue together we built a carrier board for the GPS + LNA. It fits in the side RF cages almost up to the bulkhead.
- Power comes from USB. It has an FT232RL chip connected to serial port A.
- The 3.3V power is supplied from a TPS63001 switch mode power supply.
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.
- What's the part number of the SAW filters?
- Does the Crescent really talk USB? If so, how? ttyUSB?
- No, the USB was there for the manufacturer, there is no support. FTDI instead
- Where is the short cable that connects the Crescent to the LNA?
- Can we fit the Crescent and the LNA all the way up to the RF bulkhead?
- I think so. See carrier board