grGPS
The goal of this project is to design a single board receiver that has an RF input at on end, dedicated to picking up the GPS L1 Signal, and a Baseband interface at the other end, with baseband samples heading for demodulation. The intent here is to hook up to the GNU Radio USRP for the baseband demodulation and signal processing.
- GPS System Parameters
- GPS L1 Center: 1575.42 MHz
- GPS L2 Center: 1227.6 MHz
- GPS Bandwidth: C/A 2.046 MHz
- GPS Bandwidth: P(Y) 20.46 MHz
GPS ASICS
Here are the ASIC's that I've found from perusing the internet and GPS World Magazine. I'm sure there are more available, but getting parts is difficult for the midnight engineer.
Freescale
- MRFC 1520
The 1502 seems to be an older, less integrated part. This also has an integrated PLL, but it draw's 50 mA at 5 V. This also requires an external A/D Converter. - MRFIC 1504
The 1504 is in integrated PLL part, with an external loop filter. The reference frequency is a common for GPS 16.368 MHz crystal. There is no support for an external LNA, and the IF Filter is external.
NEC
- UBP 1008K
Maxim
- MAX 2742
Replaced by the Max 2745 - MAX 2745
This seems to be the newest part that Maxim has. Runs from a 16.368/32.736 MHz crystal, with temperature compensation on chip. The output is digital, and there is a supply for an external LNA. - MAX 2741
Seems to be a newer part than the 2745. While the 2745 required a special crystal frequency, the 2741 can use crystals that are multiples of 200 kHz, something that is pretty straightforward to find.
Schematic Design
Here is the latest version of the schematic. PDF. This is still missing several items before it's ready for board layout.
Memory
Each USRP Daughter board has a small EEPROM on board to identify what the receiver board is. I need to determine the part footprint, build it in Eagle, and put it on the board.
GPS Band Filter
It's my old school RF background coming out, but I'd rather not have a product that presents a bare LNA to the antenna. Again, some distributor surfing is in order.
Board Design
Looking at the schematic, it's possible that a 2-layer board would work, since there aren't a lot of components on the board. However, I'll use a 4 layer board so that I can have lots of ground plane on the board. I'm going back and forth on putting a can over the RF stuff. I can see advantages and disadvantages both ways.
Other GPS Information
There are two other Global Positioning systems worth discussing. The first is GLONASS, the Russian system. The other in Galileo, which is a European system that is in the planning stages.
One of the better sources out there for up to date information on GPS is GPS World Magazine. Light on the detailed engineering sometimes, but lot's of coverage of the applications side of using GPS. I've got a print subscription, but they offer a digital subscription as well. Link. Some of the most interesting information to me is discussion of the politics behind some of the decisions that are made.
Other interesting web sites