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WARNING: The gpsd has been obsoleted by the ogpsd which offers a DBUS interface. Do not run these two daemons side by side. If backward compatibility is needed, use ogpsd with fso-gpsd.


What is GPS

The Global Positioning System (GPS) is a a satellite positioning system, using a constellation of 31 satellites.

GPS uses these as "man-made stars" to calculate positions to within a meter. With some forms of GPS. measurements accurate to better than a centimeter can be made.

With advancing technology, receivers have shrunk from several dozen ICs and hundreds of other components, to one, and a handfull.

This has drastically reduced costs.

The reducing cost has enabled it to be easily integrated in phones, laptops, cameras, as well as more traditional apparatus such as farming equipment, navigation systems and construction equipment.

(Another helpful overview Global Positioning System Overview)


is described on a page of its own.

PMB 2520 Hammerhead

The PMB 2520 Hammerhead is a one-chip solution for GPS that is produced by Infineon Technologies in cooperation with Global Locate. It allows the usage of assistance data by supporting A-GPS standards.

(all infomation is coming from the datasheet of PMB 2520) short datasheet

Modules of the Hammerhead

The Hammerhead consists of the following modules:

  • RF front-end with on-chip, high gain and low noise, LNA, I/Q mixers, on-chip polyphase complex IF filter, digitally controlled AGC, and 3-bits ADC for the I and Q paths.
  • Sigma-Delta RF PLL with on-chip PCO and on-chip loop filter.
  • Embedded PLL and NCO for baseband clock generation.
  • Multiple channels digital mixers and parallel correlator engines to enable real time correlation of the PRN code for up to 14 satellites.
  • Post processor including peak detection logic
  • SRAM for storing correlation results

Host Interface

The Hammerhead integrates 3 serial interfaces:

  • UART
  • I2C
  • SPI

The UART in the Hammerhead is a full-duplex UART interface. It is fixed in 8N1(8 data bits, no parity, 1 stop bit) mode. On the GTA01, the host processor connect with the Hammerhead through the UART. In all models of Neo1973, this is connected to a serial port.

Host software Architecture

The Hammerhead driver software will be delivered as a binary, which can be interfaced to gpsd as it outputs NMEA information, as many serial GPSs do.

The gps driver is now available. (There were some problems in the past getting a license to distribute the binary. [1])

The gpsd libraries provide the following infomation to the high level software:

  • Position data
  • Library status
  • Time-out and Packet Available

The high level software sends the following messages to the plugin:

  • Assistance data
  • Positioning Commands
  • Configuration Commands

Gpsd communicates with the system at the following part:

  • Communications Drivers
  • System Timer
  • NV storage
  • Log buffer

GPS on GTA01

At the GTA01, the host processor is a S3C2410.

Hammerhead on the GTA01

On the GTA01, the Hammerhead are configured that connected with the host processor through the UART. The UART of data output/input is connected to the UART 1 of the SC2410. The UART of the hardware flow control is connected to the UART 2 of the SC2410.

GPS on GTA02

On Om 2009 use ogpsd

To get gpsd working on the Neo FreeRunner with Om 2008.x you have to run the following commands:

echo "GPS_DEV=\"/dev/ttySAC1\"" > /etc/default/gpsd
/etc/init.d/gpsd restart

Q & A

Q: Can gpsd support Differential GPS.

  • While the neo does not have any means of receiving DGPS or WAAS/SCCM directly, it can be streamed from an internet server.
  • It may be possible to generate a global ionospheric model from stationary (charging?) neos that have GPS signal and cheap internet connections. This would enable very precise positions to be generated
  • This could generate positions accurate to well under a metre, compared to (probably) 2-3m without.
  • The gpsd plugin is the place that these corrections would need to be done, as they need to be performed on a per-satellite basis, before generating the position.
  • This is separate from AGPS - AGPS gives information on current satellite position, or computes your position for you. DGPS is a local minute by minute 'ionospheric weather' for your region.

See also Server:A-GPS.

Q: Can someone upload somewhere an strace of the interaction between gpsd, and the hammerhead chip?

  • Ideally this would be requesting a GPS position every second, starting from 'cold', with no AGPS data, for at least half an hour, in an area where the reciever can see the sky.
  • To aid in reverse engineering efforts.
  • A reverse-engineering page has been created: Hammerhead/Protocol

Q: Can I get NMEA data from the GPS?


  • The gpsd program 'gpspipe', with the -r switch will output NMEA data with the current position information. The right way to do it is to use libgpsd in your program if possible.
  • Gllin writes its NMEA data to the pipe by default, see getting GPS console output with gllin for more.
  • On FreeRunner, you can also read directly from the GPS chip by reading the file (communication pipe) from /dev/ttySAC1.
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