Possible GPS research (hobby) platform inspired by Namuru

This work is inspired by the GPS receiver stack (hardware, gateware, software) from UNSW first published in the 2006-2007 time frame known as Namuru. Technically, that looks like a wonderfully successful project! Props to Peter Mumford, Kevin Parkinson, Andrew Dempster, and everyone else on the team at UNSW! I see four main problems with carrying the results forward to a broader open source community:

1. The GPS front end chip they used (Zarlink GP2015) is now considered obsolete, and requires an unusual (35.42 MHz) SAW filter.
2. They used a closed source soft-core CPU (NIOS-II)
3. They used closed source upper level GPS software (GPS Architect).
4. The assembled hardware they made available ended up too pricy for hobby use (over US$5000).

This nascent project attempts to address those problems:

1. Switch to a Skyworks SE4150L GPS receiver chip, which sells for US$2.99 in single-units from Digi-Key, and uses a standard-frequency (1575 MHz) SAW filter. It's a single-conversion SSB receiver, where the GP2015 is a triple-conversion receiver.
2. Switch to an open source soft-core CPU: the LM32 seems to have the most "traction" at the moment, with full GCC support.
3. Follow development of the open source gpstk and/or rtklib for the upper level GPS software.
4. Use a generic FPGA development board, and only build an add-on module with the dedicated GPS receiver hardware.

I started off with an SE4110L as the GPS receiver, only to find the message from Skyworks that the SE4110L is "Not recommended for new designs - replacement is SE4150L." Good thing I didn't plan to use a 19.5 MHz clock! Both chips are in-stock at Digi-Key, with identical prices.

The interface between the GPS module and the FPGA development board is chosen as Digilent's Pmod. A double-high Pmod interface carries eight user-I/O pins, which can carry the GPS clock, two ADC bits, and an I2C bus for housekeeping. I can find several off-the-shelf FPGA boards with such an interface:

• XuLA2-LX25 plus StickIt! from XESS. Carries an XC6SLX25 and 32 MB SDRAM. Sells for US$142 (including power supply).
• ZedBoard. Carries an XC7Z020 and 512 MB DDR3 RAM. Sells for US$395 from Avnet, or an academic version for US$319 from Digilent.
• Experimental Board 1.2 or 1.3 from ZTEX, with various FPGA daughter modules. Options range from a XC6SLX9 with 64 MByte DDR1 RAM to a XC6SLX150 with 128 MByte DDR2 RAM. Prices range from US$164 to $588. The 'LX25 option would run US$205.
• Pipistrello from Saanlima. Carries an XC6SLX45 and 64 MByte DDR1 RAM. Sells for US$150.
• Spartan-6 LX9 MicroBoard from Avnet. Carries an XC6SLX9 (ouch) and 64 MByte DDR1 RAM. Sells for US$89!

The initial configuration will have an SMA connector to which a GPS active antenna (such as is available from AdaFruit) is connected.

The essential components with which to build the GPS receiver are:

To which I add a low-noise 3.0V regulator, passives, and I2C monitoring hardware. Still, the component parts cost is only US$33 in singles, US$28.20 in 25's. Board fabrication is likely less than US$3 in moderate quantity (~50). I'm less clear on the cost of assembly. If small-volume product-ized, I can hope it would sell for under US$100.

Board design files (using gEDA) (not cleared for fab). This tentative board design is two layers, 50.8 x 20.32 mm, where the width is as required by the Pmod standard: 0.8 inch. I'm a little concerned about choosing a Pmod pin for the clock signal that routes to an FPGA GCLK pin. Then again, at 16 MHz, I probably shouldn't worry.

Before building any hardware, I'd like to see the gateware and software start to come together. Ideally, it will all fit in an LX25 so I can use the XESS board. The GPS correlators from the Namuru project are not a problem: I have a version converted to portable Verilog (no LPM instantiation) that compiles on Icarus, synthesizes on Xilinx XST, and take up 45% of the fabric of an LX25 (but I can see how to reduce that significantly). The OHWR LM32 core takes 12% of the LX25, but I'm not sure how complete this is. That should leave plenty of space to experiment with FFT-based satellite acquisition and other ideas discussed by UNSW. Open source software that might be relevant to build on:

• GNSS SDR
• gpstk
• rtklib
• OpenSourceGPS, also here
• The Essential GNSS Project
• The GPL-GPS Project
• Homemade GPS Receiver, compact and apparently complete. Uses FPGA and Raspberry Pi, code is GPL'd.
• Milkymist (for LM32 SOC Verilog and associated support)

If and when this setup works, others will be free to complain that:

• Unlike the Namuru, it doesn't include an IMU (but you're free to add a Pmod gyro and accelerometer; using that information in the correlator sounds interesting and helpful)
• The FPGA board isn't open source (unless you choose the XuLA, and even then its design software isn't open source)
• The FPGA tool chain isn't open source

But those are topics for another day.

Comments as of 2013-06-05: I haven't forgotten about this project. The board layout looks pretty much OK; the few things I might want to add are a power jumper to allow feeding 5V from a non-Pmod pin on the Pipistrello, and a test point or two. Mostly I'm trying to get to the point where I can build gateware and software to support this thing -- I really don't like building hardware that I don't have (potentially) working software for. That will also help answer questions like: do I really need external RAM, and will the design fit in an LX9?

Larry Doolittle
June 5, 2013