Difference between revisions of "Sensors/GPS"

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Paparazzi autopilots are designed around the popular [http://www.u-blox.com u-blox] brand of receivers due to their small size, excellent performance, and 4Hz position update rate.  The [[Tiny]] features an onboard LEA series receiver, while [[Classix]] and [[Previous_Autopilots|AVR-based]] boards require an external receiver such as the SAM-LS.  Receivers must be configured prior to use with the autopilot as indicated below.
The proprietary UBX protocol is used as it offers more information and efficiency than the universal NMEA protocol.  The protocol is parsed in <tt>sw/airborne/gps_ubx.c</tt>.  Other GPS brands would require a similar parsing file to be written for NMEA or other proprietary protocols.   
==GPS Receivers==
==GPS Receivers==
===u-Blox LEA Series===


We currently use and support two different GPS Receivers from [http://www.u-blox.com u-blox].
The Tiny currently uses the [http://www.u-blox.com/products/lea_4p.html u-blox LEA-4P] featuring [http://www.u-blox.com/technology/antaris4/index.html Antaris-4] technology and uses uBlox's more efficient UBX binary protocol.  This module is a surface mount package which is soldered directly onto the target board (Tiny in this case).  The Tiny also features a battery backup (capacitor) that allows the GPS to retain data while powered off for significantly faster signal re-aquisition.
 
===uBlox LEA GPS Receiver===
 
The Tiny uses the new [http://www.u-blox.com/products/lea_4p.html LEA-4P] from uBlox. It features their new [http://www.u-blox.com/technology/antaris4/index.html Antaris-4] technology and uses uBlox's more efficient UBX binary protocol.  This module is a surface mount package which is soldered directly onto the target board (Tiny in this case).


*Positional Rate of 4Hz
*4Hz Position update rate
*Supports Active or Passive Antennas
*Supports active or passive antennas
*Supports [http://en.wikipedia.org/wiki/DGPS DGPS], [http://en.wikipedia.org/wiki/WAAS WAAS], [http://en.wikipedia.org/wiki/EGNOS EGNOS], and [http://en.wikipedia.org/wiki/MSAS MSAS]
*Supports [http://en.wikipedia.org/wiki/DGPS DGPS], [http://en.wikipedia.org/wiki/WAAS WAAS], [http://en.wikipedia.org/wiki/EGNOS EGNOS], and [http://en.wikipedia.org/wiki/MSAS MSAS]


[[Image:Lea big.jpg ]] ublox LEA GPS Receiver
[[Image:Lea big.jpg|thumb|left|u-blox LEA GPS Receiver]]
[[Image:TINY_1.3_MCU_BOTTOM.JPG|thumb|left|LEA-4P installed on the Tiny]]
<br style="clear:both">


===uBlox SAM-LS GPS Smart Antenna===
===Paparazzi Stand-alone GPS Receivers===
The other boards use a stand alone module from u-blox called the SAM-LS. It is an integrated TIM-LP module with a ceramic patch antenna. This processor also runs on 4hz and is reprogrammed to run on UBX protocol. Both modules have proven reliable and robust. With battery back up they show hot starts of around a couple seconds. The LEA-LA processor weighs a couple grams, while the SAM-LS module with antenna and shielding weighs about 20grams.
There are currently two LEA-based stand-alone GPS receiver/antenna prototype boards in development, one based on the Sangshin 13mm patch antenna and another based on the Sarantel helix antenna.  Drawings are available from the [http://cvs.savannah.gnu.org/viewcvs/paparazzi/paparazzi3/hw/ CVS]
===u-Blox SAM-LS GPS Smart Antenna===
The other boards use a stand alone module from u-blox called the SAM-LS. It is an integrated TIM-LP module with a ceramic patch antenna. This processor also runs on 4hz and must be configured to use the UBX protocol. Both modules have proven reliable and robust. With battery backup (3V watch battery) they show hot starts of around a couple seconds. The LEA-LA processor weighs a couple grams and the complete the SAM-LS module with antenna and shielding weighs about 20grams.
* Note: Effective 12/2006 u-blox has begun to phase out the SAM-LS product.  No replacement has been announced.


[[Image:Ublox_SAM-LS.jpg]] ublox SAM-LS GPS Smart Antenna
[[Image:Ublox_SAM-LS.jpg|thumb|left|u-blox SAM-LS GPS Smart Antenna]]
 
<br style="clear:both">
This section should be rather short as the GPS receivers from Ublox are pretty much ready to go when you get them. Some things are different in protocol however so you will need to program them using Ublox's software.


===Sourcing from u-blox===
===Sourcing from u-blox===


uBlox keeps tight control over the distribution of their products.  They must be obtained DIRECTLY from their own reseller offices.  These offices may not be available in your area, for example Canada does not have a reseller.  Sample quantities can be obtained from uBlox but overnight or 2 day shipping is required which drives the cost up considerably.  While it is a large hassle obtaining these devices, it is undoubtedly worth it.  Check with project members to see if they have a few lying around first.
uBlox keeps tight control over the distribution of their products.  They must be obtained DIRECTLY from their own reseller offices.  These offices may not be available in your area, for example Canada does not have a reseller.  Sample quantities can be obtained from uBlox but overnight or 2 day shipping is required which drives the cost up considerably.  While it is a large hassle obtaining these devices, it is undoubtedly worth it.   


==GPS & Control Boards==
==GPS & Control Boards==
The autopilot hardware currently uses two different GPS receivers and are summarized below.


===Tiny AP===
===Tiny AP===


This board is designed to have both the GPS receiver and antenna soldered directly to it.  It accepts any receiver from the u-blox LEA. Keep in mind that if you are not using a GPS model with flash (LEA-LA) you will need to configure the AP to send boot commands to the GPS on start up.  The programmable models include Flash EEPROM internally and save their configurations indefinitely.
This board is designed to have both the GPS receiver and antenna soldered directly to it.  The LEA-LA, LEA-4P, and LEA-4H receivers have all been used with great success though any receiver from the LEA series should work. Keep in mind that if you are not using a GPS model with flash (LEA-4A) you will need to configure the AP to send boot commands to the GPS on start up.  The programmable models include Flash EEPROM internally and save their configurations indefinitely.
 
===Classix and Lite AP===
 
These boards are configured to connect to an externally mounted u-blox SAM-LS module  The SAM-LS runs at 3.3v(Lite uses a resistor bridge). The only suggestion here is to remove the case on bottom, and solder the 4 wires directly to the TIM GPS module(GND (pin 1) ,VCC (pin 2),TX (Pin 5),RX (pin 4)) u-blox has the pinout in the manual for the GPS.


===1.2.1 AVR and RoboStix AP===
===Classix, 1.2.1 AVR, Lite, and RoboStix AP===


This board requires a GPS board which includes a 3.3v regulator and a resistor bridge. The PCB files for this board are in the CVS under SAM-LS.
These boards are configured to connect to an externally mounted u-blox module such as the Paparazzi stand-alone receivers or the commercially available but obsolete SAM-LS.  The u-blox receivers require 3.3v power and all current models have 5V tolerant data lines. The best way to connect to the SAM-LS is to remove the bottom case and solder the 4 wires directly to the TIM-LL module (GND (pin 1) ,VCC (pin 2),TX (Pin 5),RX (pin 4)) check the TIM-LL datasheet for pinout diagrams.  The Classix and Lite boards feature a 3.3V regulator to power the GPS.


==GPS configuration using U-Center==
==GPS configuration using U-Center==
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* Note: You must [[Compiling#USB_flashing|install the UART tunnel]] to enable direct access to the built-in GPS on the [[Tiny|Tiny]].
* Note: You must [[Compiling#USB_flashing|install the UART tunnel]] to enable direct access to the built-in GPS on the [[Tiny|Tiny]].


The u-blox and Tiny UARTs both operate at 3.3V TTL levels and are 5V TTL tolerant.  You must use a level shifter such as the common MAX232 to connect these devices to a standard PC serial port.  The easiest and most recommended method is to connect to a USB port instead of serial by using a $20 [http://www.ftdichip.com/Products/EvaluationKits/TTL-232R.htm FTDI USB-TTL converter cable] available from Digikey, Mouser, or direct from FTDI.  Other similar converters are available from [http://www.pololu.com/products/pololu/0391/ pololu] or [http://www.sparkfun.com/commerce/product_info.php?products_id=199 sparkfun].  A stand-alone GPS such as the SAM-LS will require clean 3.3V/50mA power and a common ground with the TTL converter.
The u-blox and Tiny UARTs both operate at 3.3V TTL levels and are 5V TTL tolerant.  You must use a level shifter such as the common MAX232 to connect these devices to a standard PC serial port.  The easiest and most recommended method is to connect to a USB port instead of serial with the $20 [http://www.ftdichip.com/Products/EvaluationKits/TTL-232R.htm FTDI USB-TTL converter cable] available from Digikey, Mouser, or direct from FTDI.  Other similar converters are available from [http://www.pololu.com/products/pololu/0391/ pololu] or [http://www.sparkfun.com/commerce/product_info.php?products_id=199 sparkfun].  A stand-alone GPS such as the SAM-LS will require clean 3.3V/50mA power and a common ground with the TTL converter.


* U-blox occasionally releases firmware updates.  Log on to the u-blox website using ''paparazzi'' for username & password to view or download the latest firmware images.  There have never been any updates released for the Antaris-4 series used in the Tiny.
* U-blox occasionally releases firmware updates.  Log on to the u-blox website using ''paparazzi'' for username & password to view or download the latest firmware images.  There have 'never' been any updates released for the Antaris-4 series used in the Tiny.


Start U-center and choose your com port from the pull down list under the connect button near the top left corner of the window. Choose your baudrate from the pull down box to the right of the connect button or select the auto-baud button to the right of that.  U-blox default is 9600 baud.  This is usually changed to 19200 baud to accomodate the 4Hz update rate.
Start U-center and choose your com port from the pull down list under the connect button near the top left corner of the window. Choose your baudrate from the pull down box to the right of the connect button or select the auto-baud button to the right of that.  U-blox default is 9600 baud.  This must be changed to 19200 or higher to accomodate the 4Hz update rate.
<br>[[Image:U-center_buttons.jpg|connect, baud, and autobaud buttons]]
<br>[[Image:U-center_buttons.jpg|connect, baud, and autobaud buttons]]
<br style="clear:both">
<br style="clear:both">


===Uploading the Configuration File===
===Uploading the Configuration File===
Download the appropriate configuration file below and use u-center to load in onto your receiver. Under the tools menu, choose GPS config, then click file>>GPS.  Be sure to check the box "Save to ROM" when loading a configuration file to make the changes permanent.
Download the appropriate configuration file below and use u-center to load in onto your receiver. Under the ''Tools'' menu, choose ''GPS configuration''.  Be sure the box 'Store configuration into BBR/Flash' is checked and hit the button ''File>>GPS''.  A few errors and retries are normal, but a significant number of errors may indicate a poor connection and the software will notify you if it is unable to send all the data successfully.
* [[Media:Example.zip|LEA-4P]]
* [[Media:Tiny_LEA-4P-v5.zip|LEA-4P]]
* TIM-LL
* [[Media:Tim-LL-V5.zip|TIM-LL]]


===Manual Configuration===
===Manual Configuration===
Line 72: Line 70:
  6. UBX->CFG->RXM(Receiver Manager) - change the '''Measurement Period''' to '''250ms''' (4 Hz position updates)
  6. UBX->CFG->RXM(Receiver Manager) - change the '''Measurement Period''' to '''250ms''' (4 Hz position updates)
  7. UBX->CFG->SBAS : '''Disable''' (SBAS appears to cause occasional severe altitude calcuation errors)
  7. UBX->CFG->SBAS : '''Disable''' (SBAS appears to cause occasional severe altitude calcuation errors)
  8. UBX->NAV (not UBX->CFG->NAV): double click on '''posutm, status, svinfo, velned.''' They should change from grey to black
  8. UBX->NAV (not UBX->CFG->NAV): double click on '''POSUTM, SOL, STATUS, SVINFO, VELNED.''' They should change from grey to black
  9. UBX->CFG->CFG : '''save current config''', click '''"send"''' in the lower left corner to permanently save these settings to the receiver.  
  9. UBX->CFG->CFG : '''save current config''', click '''"send"''' in the lower left corner to permanently save these settings to the receiver  
 
* Cycle the power and verify that the new configuration was saved
* To reset the receiver to the factory defaults go to ''UBX->CFG->CFG'', select 'Revert to default configuration', and click ''Send'' at the bottom left corner.  To permanently save these values choose 'Save current configuration' and click ''Send''.
* To backup the configuration to a file on your PC: under the tools menu, choose GPS configuration, then click GPS>>file.  This file can be re-loaded in a similar manner to configure additional identical receivers.  Be sure the box 'Store configuration into BBR/Flash' is checked when reloading a configuration file to make the changes permanent.


To backup the configuration to a file on your PC: under the tools menu, choose GPS config, then click GPS>>file.  This file can be re-loaded in a similar manner to configure additional identical receivers.  Be sure to check the box "Save to ROM" when reloading a configuration file to make the changes permanent.
===DGPS (Differential GPS)===
Differential GPS is any method of improving GPS accuracy by comparing the GPS-indicated position of a nearby location to the known value and transmitting any error to the mobile unit.  DGPS was originally created as a means of bypassing the deliberately introduced inaccuracies in civilian GPS signals.  The original method used low frequency ground radios to relay correction data to the mobile unit and is still used today at airports, shipping ports, and even individual farms.  Satellite Based Augmentation System (SBAS) is a modern form of DGPS where the ground stations relay correction data to a GEO-Stationary satellite, which then relays it to the mobile unit on standard GPS frequencies eliminating the need for a separate radio reciever.  SBAS is currently available in 3 regions, [http://www.environmental-studies.de/Precision_Farming/EGNOS_WAAS__E/3E.html WAAS, EGNOS, and MSAS] though only WAAS is officially operational.  U-blox receivers support all common varieties of DGPS [http://www.u-blox.com/customersupport/gps.g3/ENGOS_Issues(GPS.G3-CS-04009).pdf read the u-blox SBAS application note].
* It is important to note that DGPS methods only improve the ''accuracy'' of the position calculation, not the ''precision''.  Since Paparazzi navigation is typically performed relative to the power-on location, any static error that could be corrected with DGPS is irrelevant.
 
====WAAS issues====
WAAS has been officially operational and "suitable for safety-of-life applications" since 2003.  The default setting of all u-blox receivers ignores WAAS correction data and only uses the WAAS satellites for regular navigation like any other satellite.  U-blox recommends further limiting this setting to exclude any stray EGNOS/MSAS satellites in North America, and completely disabling all SBAS functions for use outside North America.
====EGNOS issues====
EGNOS is officially in "testing mode" and no claims of reliability are made.  The [http://www.u-blox.com/customersupport/faq_antaris u-blox FAQ] states the following:
* "Do you see issues with EGNOS?"
*:"Yes. Although the data transmitted by the EGNOS satellites are usually good and valuable (e.g. during the solar storms in autumn 2003), they can sometimes be very unreliable, for example when system tests are performed. As an example, u-blox has noticed erroneous range information (up to three hundred kilometers) on various EGNOS satellite over the last few months [2006]."


===Further Reading===
===Further Reading===


u-blox has developed a very thorough document called the [http://www.u-blox.com/customersupport/antaris4_doc.html System Integration Manual] which covers a lot of GPS theory as well as product specific topics.  Take a look if you have the time and interest, it may not be necessary.
The u-blox [http://www.u-blox.com/customersupport/antaris4_doc.html System Integration Manual] covers a lot of GPS theory as well as product specific topics.


== Antennas ==
== Antennas ==


==== Sangshin ====
=== Sangshin 13mm Patch ===


Part of interest: '''[http://www.sangshinec.com/eng/patch_spec.htm KSA-ST1580MS13]'''
Part of interest: '''[http://www.sangshinec.com/eng/patch_spec.htm KSA-ST1580MS13]'''
Line 102: Line 113:
[http://www.systroninc.com/ Systronic INC.] - Alberta, Canada
[http://www.systroninc.com/ Systronic INC.] - Alberta, Canada


==== 20mm Salvaged "No Name" Antenna ====
=== 20mm Unknown Salvage Patch ===


Reportedly the best match for the Tiny 0.9 through 1.1.  No source for this antenna has been found yet.
Reportedly the best match for the Tiny 0.9 through 1.1.  No source for this antenna has been found yet.
Line 110: Line 121:
'''Availability'''
'''Availability'''


These are salvaged parts and turn up from time to time in our group.  Check around.
These antennas were salvaged from a batch of otherwise defective GPS/GSM car antennas.  Availability is very limited.  Manufacturing trends appear to be moving away from 20mm antennas and toward 18mm.


==== [http://www.transplantgps.com EMTAC] 18mm ====
=== [http://www.transplantgps.com EMTAC] 18mm ===


The part number for the 1580MHz 18x18mm is ANA1580T18D40 and is not listed on their website.   
The part number for the 1580MHz 18x18mm is ANA1580T18D40 and is not listed on their website.   
Line 122: Line 133:
Direct from [http://www.transplantgps.com/modules.html TransplantGPS] in MN, USA.  The antennas should be in stock at a cost of $3.55ea but they may want you to order > $50 USD per order.
Direct from [http://www.transplantgps.com/modules.html TransplantGPS] in MN, USA.  The antennas should be in stock at a cost of $3.55ea but they may want you to order > $50 USD per order.


==== Sarantel GeoHelix-P2 ====
=== Sangshin 18mm Patch ===
Sangshin antennas are available from [http://www.rfmw.com rfmw]
 
=== Sarantel GeoHelix-P2 ===


[http://www.sarantel.com/products/geohelix-p2 GeoHelix-P2] Passive GPS Antenna [[http://www.sarantel.com/downloads/specifications/geohelix-p2.pdf datasheet]]
[http://www.sarantel.com/products/geohelix-p2 GeoHelix-P2] Passive GPS Antenna [[http://www.sarantel.com/downloads/specifications/geohelix-p2.pdf datasheet]]

Revision as of 19:23, 27 January 2007

Paparazzi autopilots are designed around the popular u-blox brand of receivers due to their small size, excellent performance, and 4Hz position update rate. The Tiny features an onboard LEA series receiver, while Classix and AVR-based boards require an external receiver such as the SAM-LS. Receivers must be configured prior to use with the autopilot as indicated below. The proprietary UBX protocol is used as it offers more information and efficiency than the universal NMEA protocol. The protocol is parsed in sw/airborne/gps_ubx.c. Other GPS brands would require a similar parsing file to be written for NMEA or other proprietary protocols.

GPS Receivers

u-Blox LEA Series

The Tiny currently uses the u-blox LEA-4P featuring Antaris-4 technology and uses uBlox's more efficient UBX binary protocol. This module is a surface mount package which is soldered directly onto the target board (Tiny in this case). The Tiny also features a battery backup (capacitor) that allows the GPS to retain data while powered off for significantly faster signal re-aquisition.

  • 4Hz Position update rate
  • Supports active or passive antennas
  • Supports DGPS, WAAS, EGNOS, and MSAS
u-blox LEA GPS Receiver
LEA-4P installed on the Tiny


Paparazzi Stand-alone GPS Receivers

There are currently two LEA-based stand-alone GPS receiver/antenna prototype boards in development, one based on the Sangshin 13mm patch antenna and another based on the Sarantel helix antenna. Drawings are available from the CVS

u-Blox SAM-LS GPS Smart Antenna

The other boards use a stand alone module from u-blox called the SAM-LS. It is an integrated TIM-LP module with a ceramic patch antenna. This processor also runs on 4hz and must be configured to use the UBX protocol. Both modules have proven reliable and robust. With battery backup (3V watch battery) they show hot starts of around a couple seconds. The LEA-LA processor weighs a couple grams and the complete the SAM-LS module with antenna and shielding weighs about 20grams.

  • Note: Effective 12/2006 u-blox has begun to phase out the SAM-LS product. No replacement has been announced.
u-blox SAM-LS GPS Smart Antenna


Sourcing from u-blox

uBlox keeps tight control over the distribution of their products. They must be obtained DIRECTLY from their own reseller offices. These offices may not be available in your area, for example Canada does not have a reseller. Sample quantities can be obtained from uBlox but overnight or 2 day shipping is required which drives the cost up considerably. While it is a large hassle obtaining these devices, it is undoubtedly worth it.

GPS & Control Boards

Tiny AP

This board is designed to have both the GPS receiver and antenna soldered directly to it. The LEA-LA, LEA-4P, and LEA-4H receivers have all been used with great success though any receiver from the LEA series should work. Keep in mind that if you are not using a GPS model with flash (LEA-4A) you will need to configure the AP to send boot commands to the GPS on start up. The programmable models include Flash EEPROM internally and save their configurations indefinitely.

Classix, 1.2.1 AVR, Lite, and RoboStix AP

These boards are configured to connect to an externally mounted u-blox module such as the Paparazzi stand-alone receivers or the commercially available but obsolete SAM-LS. The u-blox receivers require 3.3v power and all current models have 5V tolerant data lines. The best way to connect to the SAM-LS is to remove the bottom case and solder the 4 wires directly to the TIM-LL module (GND (pin 1) ,VCC (pin 2),TX (Pin 5),RX (pin 4)) check the TIM-LL datasheet for pinout diagrams. The Classix and Lite boards feature a 3.3V regulator to power the GPS.

GPS configuration using U-Center

u-center configuration software

U-Center is a very comprehensive Windows freeware program intended for the configuration and evaluation of u-blox receivers.

  • Download u-center (password protected download, use paparazzi for username & password)

The u-blox and Tiny UARTs both operate at 3.3V TTL levels and are 5V TTL tolerant. You must use a level shifter such as the common MAX232 to connect these devices to a standard PC serial port. The easiest and most recommended method is to connect to a USB port instead of serial with the $20 FTDI USB-TTL converter cable available from Digikey, Mouser, or direct from FTDI. Other similar converters are available from pololu or sparkfun. A stand-alone GPS such as the SAM-LS will require clean 3.3V/50mA power and a common ground with the TTL converter.

  • U-blox occasionally releases firmware updates. Log on to the u-blox website using paparazzi for username & password to view or download the latest firmware images. There have 'never' been any updates released for the Antaris-4 series used in the Tiny.

Start U-center and choose your com port from the pull down list under the connect button near the top left corner of the window. Choose your baudrate from the pull down box to the right of the connect button or select the auto-baud button to the right of that. U-blox default is 9600 baud. This must be changed to 19200 or higher to accomodate the 4Hz update rate.
connect, baud, and autobaud buttons

Uploading the Configuration File

Download the appropriate configuration file below and use u-center to load in onto your receiver. Under the Tools menu, choose GPS configuration. Be sure the box 'Store configuration into BBR/Flash' is checked and hit the button File>>GPS. A few errors and retries are normal, but a significant number of errors may indicate a poor connection and the software will notify you if it is unable to send all the data successfully.

Manual Configuration

If you prefer to setup your receiver manually or have a model not listed above, here are instructions to configure your receiver in u-center. Open the message window (menu View->messages view) to start the configuration process by changing the following settings: 

1. Right Click on the NMEA Icon and choose disable child
2. Choose UBX->CFG->NAV2(Navigation 2) - set it to use Airborne 4G (tells the Kalman filter to expect significant changes in direction)
3. UBX->CFG->PRT - set USART1 to 19200bps (must match the value in your Airframe file)
4. Change the baudrate of U-Center to 19200bps if the connection is lost at this point
5. UBX->CFG->RATE(Rates) - change GPS Mode to 3 - Auto (Enabling faster bootup only if signal levels are very good)
6. UBX->CFG->RXM(Receiver Manager) - change the Measurement Period to 250ms (4 Hz position updates)
7. UBX->CFG->SBAS : Disable (SBAS appears to cause occasional severe altitude calcuation errors)
8. UBX->NAV (not UBX->CFG->NAV): double click on POSUTM, SOL, STATUS, SVINFO, VELNED. They should change from grey to black
9. UBX->CFG->CFG : save current config, click "send" in the lower left corner to permanently save these settings to the receiver
  • Cycle the power and verify that the new configuration was saved
  • To reset the receiver to the factory defaults go to UBX->CFG->CFG, select 'Revert to default configuration', and click Send at the bottom left corner. To permanently save these values choose 'Save current configuration' and click Send.
  • To backup the configuration to a file on your PC: under the tools menu, choose GPS configuration, then click GPS>>file. This file can be re-loaded in a similar manner to configure additional identical receivers. Be sure the box 'Store configuration into BBR/Flash' is checked when reloading a configuration file to make the changes permanent.

DGPS (Differential GPS)

Differential GPS is any method of improving GPS accuracy by comparing the GPS-indicated position of a nearby location to the known value and transmitting any error to the mobile unit. DGPS was originally created as a means of bypassing the deliberately introduced inaccuracies in civilian GPS signals. The original method used low frequency ground radios to relay correction data to the mobile unit and is still used today at airports, shipping ports, and even individual farms. Satellite Based Augmentation System (SBAS) is a modern form of DGPS where the ground stations relay correction data to a GEO-Stationary satellite, which then relays it to the mobile unit on standard GPS frequencies eliminating the need for a separate radio reciever. SBAS is currently available in 3 regions, WAAS, EGNOS, and MSAS though only WAAS is officially operational. U-blox receivers support all common varieties of DGPS read the u-blox SBAS application note.

  • It is important to note that DGPS methods only improve the accuracy of the position calculation, not the precision. Since Paparazzi navigation is typically performed relative to the power-on location, any static error that could be corrected with DGPS is irrelevant.

WAAS issues

WAAS has been officially operational and "suitable for safety-of-life applications" since 2003. The default setting of all u-blox receivers ignores WAAS correction data and only uses the WAAS satellites for regular navigation like any other satellite. U-blox recommends further limiting this setting to exclude any stray EGNOS/MSAS satellites in North America, and completely disabling all SBAS functions for use outside North America.

EGNOS issues

EGNOS is officially in "testing mode" and no claims of reliability are made. The u-blox FAQ states the following:

  • "Do you see issues with EGNOS?"
    "Yes. Although the data transmitted by the EGNOS satellites are usually good and valuable (e.g. during the solar storms in autumn 2003), they can sometimes be very unreliable, for example when system tests are performed. As an example, u-blox has noticed erroneous range information (up to three hundred kilometers) on various EGNOS satellite over the last few months [2006]."

Further Reading

The u-blox System Integration Manual covers a lot of GPS theory as well as product specific topics.

Antennas

Sangshin 13mm Patch

Part of interest: KSA-ST1580MS13

Sangshin 13mm onboard.jpg

The Tiny 0.99 (not 0.9) and 1.1 were designed for this antenna however this part has not been verified for best performance yet.

Size: 13 x 13 mm
Center Frequency: 1580 MHz
Bandwidth: 5 MHz
@Fo: -15 dB
GAIN (dBi): 0 dBi
Ground Plane: 50 x 50 mm

Available From

Systronic INC. - Alberta, Canada

20mm Unknown Salvage Patch

Reportedly the best match for the Tiny 0.9 through 1.1. No source for this antenna has been found yet.

Salvaged 20mm onboard.jpg

Availability

These antennas were salvaged from a batch of otherwise defective GPS/GSM car antennas. Availability is very limited. Manufacturing trends appear to be moving away from 20mm antennas and toward 18mm.

EMTAC 18mm

The part number for the 1580MHz 18x18mm is ANA1580T18D40 and is not listed on their website.

Emtec 20mm.jpg

Availability

Direct from TransplantGPS in MN, USA. The antennas should be in stock at a cost of $3.55ea but they may want you to order > $50 USD per order.

Sangshin 18mm Patch

Sangshin antennas are available from rfmw

Sarantel GeoHelix-P2

GeoHelix-P2 Passive GPS Antenna [datasheet]

Geohelix-p2.jpg

This antenna apparently works great with the u-blox series GPS receivers so is not tied to any version of the board. It does not mount directly to the board. Impedence is 50ohm so almost any radio coax will work. Remember the antenna is passive with no RF amplification done at the antenna so every inch of extra coax introduces loss. Adding a ground plane (tin foil even) near the base of the antenna with a target size of 50mm x 50mm should also increase the performance.

Availability

Direct from Sarantel @ cost of approx $18 USD each

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