wiki:gps

Global Navigation Satellite Systems (GNSS)

GNSS (Global Navigation Satellite System) is a satellite system that is used to pinpoint the geographic location of a receiver anywhere in the world.

Each of the following GNSS systems employ a constellation of orbiting satellites working in conjunction with a network of ground stations:

  • GNSS systems are currently in operation:
    • the United States' NAVSTAR Global Positioning System (GPS)
    • the Russian Federation's Global Orbiting Navigation Satellite System (GLONASS)
  • GNSS systems not currently at full capacity:
    • Europe's Galileo is slated to reach full operational capacity by 2020

The following are similar navigation systems that do not have world-wide coverage:

  • China's BeiDou? Navigation Satellite System
  • India's GPS Aided GEO Augmented Navigation (GAGAN) and Indian Regional Navigation Satellite System (IRNSS)

References:

United States NAVSTAR Global Positioning System (GPS)

The United States NAVSTAR Global Positioning System (GPS) operates at a frequency of 1575.42MHz referred to as L1C/A. It has world-wide coverage originally using 24 satellites which began deployment in 1973 and became fully operational in 1995.

Reference:

Russian Federation Global Orbiting Navigation Satellite System (GLONASS)

The Russian Federation Global Orbiting Navigation Satellite System (GLONASS) operates at a frequency of 1602MHz + k*562.5 (where k = -7,...,5,6) referred to as L1OF. It is a space-based system providing an alternative to US GPS. Deployment began in 1982 and was originally completed in 1995 but followed with a decline in capacity until fully restored to 24 satellites in 2011.

Reference:

Satellite-Based Augmentation System (SBAS)

A Satellite-Based Augmentation System (SBAS) is a system that supports wideo-area or regional augmentation of positioning systems.

There are several common SBAS implementations:

Software Links

For using the GPS in software, please see the Operating Systems pages:

Gateworks on-board GNSS Hardware

Gateworks has several products that have on-board GNNS modules.

The advantage of using a Gateworks product with an on-board modules are:

  • Active Antenna support (with auto-short-circuit detect/protect/recovery for some models)
  • PPS signal for time synchronization
  • tighter integration

Refer to the individual product manuals for details of connection such as:

  • data serial port
  • PPS GPIO
  • Active antenna detect/protect/recovery support

Hardware UART and Device Mappings

The GPS devices used on Gateworks products communicate with the host processor over a serial UART:

  • Ventana:
    • GW5100: /dev/ttymxc0 Wi2Wi W2SG0008i 4800bd NMEA 0183 output 1Hz
    • GW52xx / GW53xx / GW54xx : /dev/ttymxc4 Wi2Wi W2SG0008i 4800bd NMEA 0183 output 1Hz
    • GW553x: /dev/ttymxc3 u-blox EVA-M8M 9600bd NMEA 0183 output of GGA/GLL/GSA/GSV/RMC/VTG/TXT GPS, GLONASS, and SBAS enabled
  • Laguna: Trimble Condor
    • GW2391: /dev/ttyS2 (Gateworks 'specials' can alternately map this /dev/ttyS1)
    • GW2388: /dev/ttyS2 (Gateworks 'specials' can alternately map this /dev/ttyS1)
    • GW2380: /dev/ttyS1
  • Avila / Cambria: Trimble Copernicus

Hardware Devices Used

u-blox EVA-M8M-0 (GW553x)

References

Features:

  • 72-channel u-blox M8 concurrent position receiver engine supporting multiple concurrent Global Navigation Satellite System's (GNSS):
    • GPS: L1C/A (1575.42MHz)
    • GLONASS: L1OF (1602MHz)
    • QZSS: L1C/A (1575.42MHz)
    • SBAS: WAAS/EGNOS/MSAS: L1C/A (1575.42MHz)
  • Differential GPS (DGPS): RTCM 10402.3:
  • Navigation Update Rate: 10Hz (GPS&GLONASS) or 18Hz (GPS)
  • Accuracy:
    • Velocity: 0.05m/s
    • Heading: 0.3degrees
    • Horiz position: Autonomous 2.5m / SBAS 2.0m
    • PPS: 30ns RMS, 99% 60ns
  • Sensitivity:
    • Tracking & Navigation: -164dBm (GPS&GLONASS) -163dBm (GPS)
    • Re-acquisition: -159dBm
    • Cold start: -147dBm
  • Acquisition:
    • Cold start Time-to-First-Fix (TTFF): 27s (GPS&GLONASS) or 30s (GPS only)
    • Hot start Time-to-First-Fix (TTFF): 1s
  • PPS:
    • configurable rate: 0.25Hz to 10Mhz
    • configurable polarity
    • configurable pulse-width: 232 micro-seconds or duty-cycle
    • Note that for best PPS accuracy it is recommended to disable the SBAS subsystem
  • Communication:
    • 9600 baud, 8bit no parity, 1 stop bit, no flow control supporting multiple protocols:
      • NMEA: Input / Output ASCII 0183 version 4.0 (configurable for 2.3 or 4.1)
      • UBX: Input / Output binary u-blox proprietary
      • RTCM: Input, messages 1,2,3,9
  • Active Antenna: 15dB to 50dB recommended
  • Power Consumption (not including active antenna power consumption):
    • ~27mA (90mW) during acquisition for GPS & GLONASS, 22mA for GPS/QZSS/SBAS
    • ~25mA (83mW) during continuous mode tracking for GPS & GLONASS, 19mA for GPS/QZSS/SBAS
    • ~5.5mA (18mW) during 1Hz power-save mode tracking (for GPS/QZSS/SBAS only)

Power-on Defaults:

  • 9600 baud, 8 data bits, no parity, 1 stop bit:
  • GPS & GLONASS enabled
  • SBAS enabled
  • output: NMEA: GGA, GLL, GSA, GSV, RMC, VTG, TXT messages
  • input: UBX, NMEA, RTCM
  • PPS: 100ms active high pulse at 1Hz

Note that the EVA-M8M is not connected to an RTC, battery backup, or external flash on Gateworks designs (which disallows for permanent configuration storage and firmware upgrade)

u-blox Receiver Protocol (UBX)

UBX is a u-blox proprietary binary communication protocol used in their GNSS engines.

Reference:

NMEA considerations for EVA-M8M

The following need to be taken into consideration when operating in multi-GNSS mode (Note that the power-on default is GPS & GLONASS):

  • multiple sets of GSV messages will be output back to back, 1 for each system
  • multiple sets of GSA and GRS messages are output for each fix, one for each GNSS

The main Talker ID (the two-letter identifier) will vary based on GNSS configuration:

  • GPS/SBAS/QZSS - GP
  • GLONASS - GL
  • GNSS - GN (message pertaining to any type of GNSS)

Wi2Wi W2SG0008i (GW54xx-C+, GW53xx, GW52xx, GW51xx)

References:

Features:

  • CSR/SiRF SiRFStarIV GSD4e
  • 48 channel continuous tracking receiver (reports best 12 of 48):
    • GPS: L1C/A (1575.42MHz)
    • SBAS: WAAS/EGNOS/MSAS/GAGAN: L1C/A (1575.42MHz)
  • Differential GPS (DGPS)
  • Navigation Update Rate 1-5Hz
  • Acquisition:
    • Hot Start TTFF@-136dBm: 0.6sec
    • Min Acquisition Signal: -153dBm
  • Sensitivity: Tracking -163dBm / Acquisition -148dBm
  • Accuracy:
    • 3m stationary horizontal (with DGPS), 15m stationary horizontal (without DPGS)
  • Communication:
    • 8 data bits, no parity, 1 stop bit (aka 8N1 or cs8), no flow control
    • baudrate: can vary between 4800 and 115200 (see below)
    • NMEA 0183 v3.01, configurable 1 to 255 seconds between messages, supports: GGA, GLL, GSA, GSV, RMC, VTG, ZDA messages
    • OSP™ (SiRF BINARY™) proprietary protocol
  • PPS: rising edge 1Hz 4.2us (requires 5+ satellites)
  • Internal LNA Gain:
    • High Gain Mode (default) intended for use with passive antennas
    • Low Gain Mode intended for use with active antennas (that have their own LNA)

Power-on Default configuration:

  • 4800 baud
  • NMEA 0183 output 1Hz
  • LNA high gain mode

W2SG0008i (SiRFStarIV) NMEA Command Reference:

References:

Notes:

  • NMEA 4800bd 8N1 (8 data bits, no parity, 1 stop bit) is the default power-on communication mode of the W2SG0008i
  • Flow control is not used between the GPS and the host processor and thus should be disabled
  • SiRF NMEA reports a maximum of 12 satellites in its GSV messages even though more may be visible (use OSP protocol if you want more detail on the 48 channels)

Examples:

  • The proprietary PSRF100 command will change the baudrate and data format. Consult the reference manual (above) section 2-3 for more info.
    • Example: To change the baud from the power-on default 4800bd to 9600bd:
      # set tty for current baudrate and data format
      stty -F /dev/gpsdevice 4800 cs8
      # send a PSRF100 command to set the format (1=NMEA mode) baudrate (9600) and data format
      echo "\$PSRF100,1,9600,8,1,0*0D" > /dev/gpsdevice
      # set new tty baudrate for further communication
      stty -F /dev/gpsdevice 9600
      
      • 1,9600,8,1 is the data payload. Specifically, the first '1' specifies that this is a NMEA message, while the rest of the payload determines the speed of the GPS receiver.
      • ensure that /dev/gpsdevice corresponds to the correct tty above in the hardware mappings section
      • if changing the above NMEA sentence, be sure to update the checksum
  • The proprietary PSRF103 command will set the rates of the various NMEA messages. Consult the reference manual (above) section 2-6 for more info.
    • Example: To set the period of all messages to one every 1 seconds:
      # set tty for current baudrate and data format assuming power-on default of 4800bd 8N1
      stty -F /dev/ttymxc4 4800 cs8
      echo "\$PSRF103,00,00,01,01*26" > /dev/gpsdevice # GGA every 2 sec
      echo "\$PSRF103,01,00,01,01*25" > /dev/gpsdevice # GLL every 2 sec
      echo "\$PSRF103,02,00,01,01*28" > /dev/gpsdevice # GSA every 2 sec
      echo "\$PSRF103,03,00,01,01*27" > /dev/gpsdevice # GSV every 2 sec
      echo "\$PSRF103,04,00,01,01*22" > /dev/gpsdevice # RMC every 2 sec
      echo "\$PSRF103,05,00,01,01*21" > /dev/gpsdevice # VTG every 2 sec
      

One Socket Protocol (OSP), aka SiRF Binary Protocol

One Socket Protocol (OSP), also known as the SiRF BINARY™ protocol, is a binary communication protocol available with the SiRFstar family of GPS receiver products from SiRF (which was purchased by CSR). In general, there are more message types and data available when using this protocol over NMEA 0183.

References:

One Socket Protocol Interface Control Document (OSP_ICD) (CS-129291-DC-15)

Examples:

  • To switch from NMEA 4800bd 8N1 to OSP at 9600 baud, you can use the following bash script:
    #!/bin/sh
    
    # Make sure we talk to the device at its current configured data format
    # (assuming power-on default of 4800 8N1)
    stty -F /dev/gpsdevice 4800 cs8
    
    # change to mode=0 (OSP/SiRF binary) 9600bd 8N1
    echo "\$PSRF100,0,9600,8,1,0*0C" > /dev/gpsdevice
    
    # set new tty baudrate for future communication
    stty -F /dev/gpsdevice 9600
    
  • To switch from OSP at 9600bd back to NMEA 4800bd, you can use the following bash script:
    #!/bin/sh
    send_hex() {
      while [ "$1" ]; do
        printf "\x$1"
        shift
      done
    }
    
    # Make sure we talk to the device at its current configured data format
    # (assuming it has been changed to 9600bd)
    stty -F /dev/gpsdevice 9600 cs8
    
    # send MID 129: Switch to NMEA protocol
    send_hex A0 A2 00 18 > /dev/gpsdevice # start sequence and payload length (24 bytes)
    send_hex 81 02 01 01 01 01 01 01 01 01 01 01 01 01 00 01 00 01 00 01 00 00 12 C0 > /dev/gpsdevice # payload specifying baudrate and msg rates
    send_hex 01 64 B0 B3 > /dev/gpsdevice # checksum and end sequence
    
    # set new tty baudrate for future communication
    stty -F /dev/gpsdevice 4800
    
    • Note that MID 129 also specifies the period of the various NMEA messages. If changing the above data you need to also re-calculate the checksum. See here section 5.4 for more info on the MID 129 message
    • ensure that /dev/gpsdevice cooresponds to the correct tty above in the hardware mappings section
  • To enable SBAS based DGPS assuming you are already in 9600bd OSP mode:
    #!/bin/sh
    
    send_hex() {
      while [ "$1" ]; do
        printf "\x$1"
        shift
      done
    }
    
    # Make sure we talk to the device at its current configured data format
    # (assuming it has been changed to 9600bd)
    stty -F /dev/gpsdevice 9600 cs8
    
    # send MID 133: DGPS Source
    send_hex A0 A2 00 07 > /dev/gpsdevice # start sequence and payload length (7 bytes)
    send_hex 85 02 00 00 00 00 00 > /dev/gpsdevice # payload specifying DPGS Source (Table 5.43)
    send_hex 00 87 B0 B3 > /dev/gpsdevice # checksum and end sequence
    
    • when DPGS is enabled GGA NMEA messages will show a position fix indicator of 2 to indicate a DGPS fix vs 1 to indicate a GPS fix.
    • use MID 170 to set SBAS parameters if you need to change the SBAS mode or region between WAAS/EGNOS/MSAS/GAGAN

W2SG0008i LNA Gain

The W2SG0008i used on most Gateworks Ventana boards defaults on power-up to use its internal high gain LNA which is appropriate for passive antennas but not necessarily appropriate for active-antennas which have their own LNA at the antenna.

While Gateworks has tested1 and found no adverse affects in terms of GPS sensitivity and accuracy when using the GW10044 Active GPS antenna with the default power-up high-gain internal LNA your results may vary depending on your antenna and system characteristics.

If you wish to configure the Ws2G0008i internal LNA for low-gain you must do the following according to the W2SG0008i datasheet section 6.4.1:

  1. Switch GPS Communication Protocol from NMEA to OSP mode.
  2. Send Tracker Configuration Message (OSP MID 178, 02) - (Disable Internal LNA and drive GPS_EXT-LNA_EN signal)
  3. Wait for SiRFStarIV ACK
  4. Perform a Hot Start reset; Tracker Configuration setting requests in message (OSP MID 178, 02) will apply after performing this reset.
  5. Wait for SiRFStarIV ACK
  6. Switch GPS Communication Protocol back to NMEA.

The gateworks-gps-utils.tar.gz attached at the bottom of this page contains source for a C application "sirf_osp" that will do the above in a short series of commands. After building the application for your BSP run the following commands to put the gps module into LNA low gain mode:

DEVICE="/dev/ttymxc4" # Refer to Hardware UART Mappings section above to verify device
./sirf_osp $DEVICE 4800 osp 115200  # Changes board from default NMEA/4800 to OSP/115200
./sirf_osp $DEVICE 115200 lna low   # Sends OSP command for LNA low mode
./sirf_osp $DEVICE 115200 config    # Prints configuration to verify LNA mode of low ("LNA:1")

1 See Indoor GPS Testing section below for test details and complete sirf_osp usage example

Trimble Condor 68674-00 C1011 (GW2380, GW2382, GW2383, GW2387, GW2388, GW2389, GW2391)

References:

Features:

  • 22-channel continuous tracking receiver:
    • GPS: L1C/A (1575.42MHz)
    • SBAS: WAAS: L1C/A (1575.42MHz) (disabled by default)
  • Differential GPS (DGPS)
  • Acquisition: cold start - 38 sec 50%
  • Sensitivity: Tracking -160dBm / Acquisition -146dBm
  • Accuracy:
    • Horizontal w/o SBAS: <2.5m 50%, <5m 90%
    • Horizontal w/ SBAS: <2.0m 50%, <4m 90%
    • Altitude w/o SBAS: <5m 50%, <8m 90%
    • Altitude w/ SBAS: <3m 50%, <5m 90%
    • Velocity: 0.06m/sec
    • PPS: +25ns 1 sigma
  • Communication:
    • 8 data bits, no parity, 1 stop bit (aka 8N1 or cs8), no flow control
    • baudrate: can vary between 4800 and 115200 - refer to user guide for details on changing baudrate
    • NMEA 0183 v3.0, 1Hz - 5Hz NMEA update rate, supports: GLL, RMC, VTG, GGA, GSA, GSV, ZDA messages
    • TSIP (Trimble Standard Interface Protocol)
    • TAIP (Trimble ASCII Interface Protocol)
  • PPS: rising edge

Power-on Default configuration:

  • 9600 baud
  • output NMEA RMC, GGA, GSV, and GSA messages 1 time for every position fix (1Hz)

NMEA Configuration

The types of NMEA messages and their rates can be configured by sending PMTK messages.

Refer to the User guide for a full listing.

Notes:

Common configuration items:

  • PMTK251 - Set NMEA baud rate
  • PMTK313 - Set SBAS enable
  • PMTK314 - Set NMEA sentence types and frequency (only RMC, GGA, GSV, and GSA are enabled by default)

Examples:

  • To turn on VTG messages at 1Hz (course over ground and ground speed) (disabled by default):
    stty -F /dev/gpsdevice 4800 cs8
    echo -n -e "\$PMTK314,1,1,1,1,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0*28\r\n" > /dev/gpsdevice
    

Trimble Copernicus 63530-00 (GW2350, GW2355, GW2358, GW16033, GW16039)

References:

Features:

  • 12-channel continuous tracking receiver:
    • GPS: L1C/A (1575.42MHz)
    • SBAS: WAAS/EGNOS/MSAS: L1C/A (1575.42MHz)
  • Differential GPS (DGPS)
  • Acquisition: cold start - 38 sec 50%
  • Sensitivity: Tracking -160dBm / Acqusition -142dBm
  • Accuracy:
    • Horizontal w/o SBAS: <2.5m 50%, <5m 90%
    • Horizontal w/ SBAS: <2.0m 50%, <4m 90%
    • Altitude w/o SBAS: <5m 50%, <8m 90%
    • Altitude w/ SBAS: <3m 50%, <5m 90%
    • Velocity: 0.06m/sec
    • PPS: +60ns RMS
  • Communication:
    • 8 data bits, no parity, 1 stop bit (aka 8N1 or cs8), no flow control
    • baudrate: can vary between 4800 and 115200 - refer to user guide for details on changing baudrate
    • NMEA 0183 v3.0, 1Hz - 5Hz update rate, supports: ZDA, GGA, GLL, VTG, GSA, GSV, RMC messages (output in that order)
    • TSIP (Trimble Standard Interface Protocol)
    • TAIP (Trimble ASCII Interface Protocol)
  • PPS:
    • rising edge 1Hz 61ns
    • requires fix interval set to 1000ms or less - see datasheet for details

Power-on Default configuration:

  • 4800 baud
  • output NMEA GGA, VTG messages 1 time for every position fix (1Hz)

Antennas

GPS receivers can use active or passive GPS antennas. An active antenna is powered and uses an LNA (Low Noise Amplifier) at the antenna instead of at the GPS receiver making it more sensitive than a typical passive antenna.

Some of the GPS modules can be used with passive antenna. See the modules datasheet for more information. Gateworks provides 3.3V for the active antenna DC voltage on all boards.

Gateworks typically uses a 50 ohm MMCX connector on it's board however as a special configuration a U.FL connector can be loaded in the same location.

Here is an SMA antenna we sell in our shop that is 50 Ohm and 27-28 dBi (+- 3-4dBi) - GW10044:

Antenna Adapter Cables:

u-blox EVA-M8M (Ventana product family)

Maximum Antenna Gain:

  • 15dB to 50dB recommended

Please also consult the manual / datasheet above.

Wi2Wi module (Ventana product family)

Maximum Antenna Gain:

  • In high gain mode, a passive antenna acts as input. Total RF gain (sum of internal LNA gain, cable and filter losses) of ≤ 5 dB is considered acceptable.
  • In low gain mode, an active antenna acts as input. Total RF gain (sum of external antenna gain, internal LNA gain, cable and filter losses) of 14 to 24 dB is considered acceptable.

Please also consult the manual / datasheet above.

Trimble Condor (Laguna product family)

Maximum Antenna Gain:

  • Passive: Does not support
  • Active: 36dB

Please also consult the manual / datasheet above.

Trimble Copernicus (Avila / Cambria product family)

Maximum Antenna Gain:

  • Passive: Does not support
  • Active: 36dB

Please also consult the manual / datasheet above.

Active Antenna Short Circuit Detect / Recover

All Gateworks boards with on-board GPS support active (powered) GPS antenna connections. On many boards that have a Gateworks System Controller the antenna power is periodically monitored (1Hz) for short circuit and automatically disabled/re-enabled to allow resolving the GPS antenna without power cycling the unit.

See the product manual for your board to determine if it has this capability.

Add-in cards supporting GPS

There are many GPS receivers that communicate over UART or USB-UART. Some of these are in the miniPCIe form-factor which provides USB to the connector (Note that USB is not routed to all Gateworks miniPCIe connectors - consult the product user manual for details on your board).

References:

NMEA 0183

NMEA 0183 is a combined electrical and data specification for communication between marine electronics such as echo sounder, sonars, anemometer, gyrocompass, autopilot, GPS receivers and many other types of instruments. It has been defined by, and is controlled by, the National Marine Electronics Association. It replaces the earlier NMEA 0180 and NMEA 0182 standards.[1] In marine applications, it is slowly being phased out in favor of the newer NMEA 2000 standard.

Most GPS receivers use this standard for communication, however typically they may only use a subset of the standard, add their own proprietary commands, and use different baudrates. Refer to the specific GPS receiver documentation being used for details.

Most GPS devices allow customizing the NMEA output such as the UART baudrate, the types of messages, and the rate at which they are sent. Please see sections above for the particular GPS device you are interested in configuring

NMEA messages end with a checksum and most devices will ignore messages with an invalid checksum. You can use the online NMEA sentence checksum calculator to aid you.

References:

Time synchronization via PPS (Pulse-Per-Second)

One handy feature of an on-board GPS is that it can deliver Pulse-Per-Second (PPS) signal. This signal can be used to get a high-precision time reference that an application can use to adjust system clock time. Common use is to configure the Network Time Protocol Daemon (NTPD) with a PPS source to obtain a wallclock-time with sub-millisecond synchronization to UTC.

Our 14.08 BSP for both Laguna and Ventana families includes PPS support already (if a GPS is present). However, in order to hook NTPD with a PPS source, you have to have the fully featured ntpd package (i.e. not busybox). Below is an example of how to configure ntpd to use GPS and use PPS to keep syncronization:

  • Make sure you have the fully featured ntpd package via selecting ntpd in OpenWrt/kernelconfig.
  • Run the following steps (This is on GW5400, please see proper /dev/ttyX for your device) GPSD Time Service
    $ killall -9 gpsd ntpd
    $ ntpd -gN
    $ sleep 2
    $ gpsd -n /dev/ttymxc4 # ttymxc4 for GW5400, might be different for different boards
    $ sleep 2
    $ cgps # not needed
    
  • Note that cgps is part of the gpsd program. It is a "test client" for gpsd, though it shows very relevant information.
  • That's it! With gpsd and ntpd running, you have successfully configured your system time to come from GPS data.

For BSP's that do not include PPS by default, refer to the following guide.

Gateworks boards with on-board GPS use ARM Host GPIO lines for PPS sources. Therefore you would want to configure your kernel with:

  • CONFIG_PPS - enable PPS support
  • CONFIG_NTP_PPS - enable PPS kernel consumer (direct in-kernel time synch using PPS source)
  • CONFIG_PPS_CLIENT_GPIO - add support for a PPS source using GPIO
  • Configure "pps-gpio" driver:
    • Device-Tree Boards (ie Ventana/Newport?):
      • This is already done in the device tree for on-board GPS.
        pps {
                   compatible = "pps-gpio";
                   pinctrl-names = "default";
                   pinctrl-0 = <&pinctrl_pps>;
                   gpios = <&gpio1 26 GPIO_ACTIVE_HIGH>;
                   status = "okay";
        };
        
        pinctrl_pps: ppsgrp {
                   fsl,pins = <
                      MX6QDL_PAD_ENET_RXD1__GPIO1_IO26          0x1b0b1
                   >;
        };
         
        
    • Non Device-Tree Boards (ie Laguna):
      • Add a "pps-gpio" platform resource to the kernel board support file (ie arch/arm/mach-cns3xxx/laguna.c for Laguna)
        • see include/linux/pps-gpio.h for struct pps_gpio_platform_data
        • define gpio_pin (ie 0 for GW2388)
        • define gpio_label (ie "GPS_PPS")
        • define assert_falling_edge=0 (PPS is a rising edge)
        • define capture_clear=0 (capture PPS 'assertions')
        • make sure you comment out any other exporting of this GPIO (ie 'GPS_PPS' export from laguna_gpio_gw2388)
        • this will create an entry in /sys/class/pps/pps0

The PPS signal is also routed to a spare pin on the miniPCIe slots of many models:

Model Revision Introduced Pin PCIe Slot Resistor Loading
GW51xx Rev C 49 J6 R118
GW52xx Rev C 49 J7,J8 R190/J8 R191/J7
GW53xx Rev C 49 J7,J8 R212/J7 R213/J8
GW553x Rev A 49 J5 R31
GW54xx Rev E 49

Reference:

  • LinuxPPS
    • The Kernel side of PPS support was added in 2.6.31 - see here
  • GPSD Time Service - Lots of examples relating to gpsd
  • RFC 2783 - Pulse-Per-Second API for UNIX-like Operating Systems. This describes the Linux OS API for userland applications to access a PPS source.
  • http://gpsd.berlios.de/gpsd.html - there is some info in the gpsd manpage about PPS, time accuracy and ntp synchronization
  • Documentation/pps/pps.txt - Linux kernel API/support for synchronizing system time from a PPS source directly (Note this is the API that drivers/pps/clients/pps-gpio.c conforms to)
  • Documentation/ABI/testing/sysfs-pps - Linux sysfs pps class documentation (sysfs API for Userland apps to interface with PPS from LinuxPPS kernel support above)
  • include/linux/pps.h - Linux ioctl pps documentation/support (ioctl API for Userland apps to interface with PPS from LinuxPPS kernel support above)

Differential GPS (DGPS)

Differential Global Positioning System (DGPS) is an enhancement to GPS that provides improved location accuracy from the 15-meter nominal GPS accuracy to about 10cm in the case of the best implementations.

Radio Technical Commission for Maritime Services (RTCM)

The standards applying to Differential Global Navigation Satellite Systems (DGNS) are defined by the Special Committee 104 of the Radio Technical Commission for Maritime Services (RTCM). Except for RTCM, there exist other proprietary DGPS standards, such as Trimble Compact Measurement Record (CMR).

References:

Traditional land-based beacon DGPS

Traditional DGPS uses a network of fixed ground-based reference stations that broadcast the difference between their known location and the location determined via GPS. These stations would broadcast using VHF/UHF which traditionally were available on maritime vessels where GPS was originally intended to be used.

To use this you must have a receiver that can receive both GPS broadcasts as well as the VHF/UHF broadcasts from the land-based beacons. In addition you need to be in an area that has such land based beacons. The U.S. Coast Guard and Canadian Coast Guard operate these near large waterways.

Wide Area Augmentation System (WAAS)

An alternative system was developed called Wide Area Augmentation System (WAAS) which uses satellites in geostationary orbit that receive broadcasts from fixed ground-based stations. In this system the additional satellites periodically broadcast (about every 5 seconds) a Deviation Correction (DC) created from the data continually received from the ground based reference systems. This signal can be received with the same receiver used to receive signals from traditional GPS satellites.

To use this you need only a receiver that can receive GPS broadcasts, however you still need to be in an area that both has ground based reference stations and need to have at least 1 SBAS capable satellite in view. This system was created for North America however there are SBAS based systems being developed in Europe and Asia as well:

  • SBAS - North America / Hawaii Satellite-Based Augmentation System
  • EGNOS - European Geostationary Navigation Overlay Service
  • MSAS - Jpanase Multi-functional Satellite Augmentation System
  • GAGAN - Indian GPS Aided Geo Augmented Navigation

Indoor GPS Testing

Gateworks uses a GPS repeater when doing in-building GPS testing. These can be found from a variety of vendors:

The unit we use is the Sumalink SL1500 available for $200 and features:

  • L1 GPS: 1575±5MHz
  • 5V DC input
  • Interference suppression: >=20dB;
  • Reflection loss: -14dB
  • Range: 15 meters (re-transmission range)
  • Cable: 30 meters (cable from rooftop receiver to transmitter)

We use various test software including:

  • test_gps - shell script that parses GSV, GGA, GSA messages and provides fix, average satellites in view, and signal strength details
  • gpsd and related clients (gpxlogger can capture a path to an XML file, GPS Babel can convert GPX log to KML, Google Earth can view KML files)
  • sirf_osp - C application used for sending commands and receiving messages from ventana gps devices

An example of a test procedure carried out on our gps devices was the verification of optimal LNA mode for Ventana boards when using our standard active antenna. The procedure for this test is defined as follows:

  1. Power on the device with antenna connected
  2. Download the gateworks-gps-utils.tar.gz (see bottom attachments) and extract
  3. Use the Makefile provided to build the sirf_osp application (refer to your BSP's 'Building' wiki for help)
  4. Make the application executable chmod +x sirf_osp
  5. Put the device into osp mode ./sirf_osp <device> <baud> osp 115200
  6. Set the LNA mode of the device ./sirf_osp <device> 115200 lna <low/high>
  7. Verify the LNA mode via the "LNA:X" field of ./sirf_osp <device> 115200 config
  8. Begin storing gps fix data to file ./sirf_osp <device> 115200 csv <filename>
  9. Use gps visualization tool of your choice to analyze gathered data (e.g. Google My Maps)

*Note: If any sirf_osp command results in a stream of gps data, retrying the command with the correct arguments will typically solve the problem.

Last modified 7 weeks ago Last modified on 07/07/17 10:35:32

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