Difference between revisions of "Sensors/GPS"
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Revision as of 23:53, 24 July 2011
Overview
Paparazzi autopilots are designed around the popular u-blox brand of receivers.
- Features:
- Small size
- Excellent performance
- 4Hz position update rate
The Tiny features an onboard LEA series GPS receiver and patch antenna, while Classix and AVR-based boards require an external receiver+antenna such as the Paparazzi GPS or SAM-LS. Please note that the receivers must be configured (prior to use with the autopilot) as indicated below. Both modules have proven reliable and robust.
Note: 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 Receivers
The TWOG, Classix and AVR-based boards require an external GPS module and antenna. The Tiny features an integrated receiver and antenna. Either type is designed for u-blox 4 and 5 series GPS receivers and the proprietary UBX binary protocol. An external battery or capacitor is typically used to enable the GPS to retain data while powered off for significantly faster signal re-aquisition. Any of the LEA-4 and LEA-5 series receivers can be used including the less expensive LEA-4A, 4S, 5A and 5S models as the special boot configuration code required for these models is already written.
- 4Hz Position update rate
- Supports active or passive antennas
- Supports DGPS, WAAS, EGNOS, and MSAS
- Low position noise figure
Paparazzi Stand-alone GPS Receivers
Top
Bottom
Paparazzi source provides a design for an external GPS board. An eternal GPS board is required for Lisa, TWOG and Classix Autopilot board. Programming it is similar to the Tiny2.11 GPS configuration. If you build your own you will want to upload the latest u-blox firmware before you configure. See "Getting Hardware" for sources of assembled boards.
The Paparazzi design in http://svn.savannah.nongnu.org/viewvc/paparazzi-hardware/trunk/sensors/gps_13/?root=paparazzi. The board is very small and light as it has only the components required. It is powered from the 5v line on the "downloads" connector of a TWOG. Also note it is a 4-layer PCB that means better noise resistance. The board has pins for USB connection but requires a different cable and a solder jumper to be move from the ground (default) to 3.3v input to enable the USB port on the module.
V1 BOM.xls
Eagle Parts List Output.txt
See Get Hardware page for suppliers.
Wiring Diagram
3rd Party u-blox reference design boards:
The only other GPS board in use seems to be u-blox reference designs or similar to it. They have LEA-4H, LEA-5H and LEA-6H (typically) and several interfaces. Often a larger antenna as well.
The board in the photo is a RF DESIGN LEA-5H-SMART.
The jumpers adjacent to the TTL interface connectors need to be closed with low value resistors for paparazzi uart port use. Also a battery has to be added with an appropriate charging resistor to enable RTC functionality.
NAVILOCK NL-507ETTL
The NAVILOCK NL-507TTL u-blox TTL Modul 60416 features an LEA-4 series receiver and 25mm patch antenna on a 30mm x 30mm board.
- Datasheet: http://www.navilock.de/download/Dokumente_SLASH_Sonstiges/60415_-_Datenblatt_u-blox_GPS_Module/481
- Purchase: Available for 28€ at www.amazon.de
SPK GS406
Sparkfun sells a nice small module featuring the newer 5-series receiver and the highly rated Sarantel antenna for about $90. The design is based around the active version of the Sarantel instead of the more appropriate passive model and there's some potentially tricky soldering involved to get around the ribbon cable but the price is great for this hardware.
DIYDrones has build a adapter board for this GPS module. It looks like a great solution to use this GPS module with Paparazzi.
u-Blox C04-6H Reference Design
u-Blox sells a complete module with antenna for around $200 and will also provide complete schematics, BOM, and PCB files for free if you wish to make your own. Two versions are offered, one with an 18mm patch antenna and the other with the Sarantel P2 helical antenna.
See http://www.ublox.com/en/evaluation-tools-a-software/reference-designs/for-gps-chips/c04-6h.html for more info.
Connecting external receivers to Classix, 1.2.1, Lite, and RoboStix boards
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.[1] The Classix and Lite boards feature a 3.3V regulator to power the GPS. To open the casing on a SAM-LS, remove the bottom of the casing by pulling gently, then work around popping off the solder joints (they are fairly weak) by pressing a small screwdriver against each tab in turn until it pops off. You should then be able to access the GPS chip directly.
GPS configuration using U-Center
U-Center is a very comprehensive freeware program intended for the configuration and evaluation of u-blox receivers.
- Note 1: You must install the UART tunnel to enable direct access to the built-in GPS on the Tiny.
- Note 2: You will need a driver for your FTDI cable if you run u-center on Windows, which can be found here.
- Note 3: You can run u-center on Linux by installing "Wine" (Installation of Wine) and set up COM1 as /dev/ttyUSB0. You need to create a symbolic link from the COM device to TTY like this:
ln -s /dev/ttyUSB0 ~/.wine/dosdevices/COM1
or what worked in Ubuntu 9.10
ln -s /dev/ttyUSB0 ~/.wine/dosdevices/com1
This command will create the symbolic link from ttyUSB0 to COM1. See Info on Wine for "dosdevices" setup. Just download the u-setup.exe and run it with Wine, follow prompts. This has been tested with Ubuntu7.10 and Ubuntu 8.04 so far.
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 / 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.
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:
LEA-4P
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 38400bps (must match the value in your Airframe file) 4. Change the baudrate of U-Center to 38400bps if the connection is lost at this point 5. UBX->CFG->RXM(Receiver Manager) - change GPS Mode to 3 - Auto (Enabling faster bootup only if signal levels are very good) 6. UBX->CFG->RATE(Rates) - 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
LEA-5H
1. Right Click on the NMEA Text on top of the tree and choose disable child messages 2. Choose UBX->CFG->NAV5(Navigation 5) - set it to use Airborne 8 <4G. This tells the Kalman filter to expect significant changes in direction. Note that this setting is only good for faster moving airplanes. For a fixed position hovering heli, 'pedestrian' setting is better 3. UBX->CFG->PRT - set USART1 to 38400bps (must match the value in your Airframe file) 4. Change the baudrate of U-Center to 38400bps if the connection is lost at this point 5. UBX->CFG->RATE(Rates) - change the Measurement Period to 250ms This gives a 4 Hz position update since 4 x 250ms is one second. 6. UBX->CFG->SBAS : Disable (SBAS appears to cause occasional severe altitude calcuation errors) 7. UBX->NAV (not UBX->CFG->NAV): double click on POSLLH, SOL, STATUS, SVINFO, VELNED. They should change from grey to black (add the flag -DGPS_USE_LATLONG in your Airframe file) also make sure you set tiny_2_1_1.h if you have the latest boards Tiny/TinyWOG) 8. 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 update the firmware on a LEA-5H get u-center >= 5.03, revert the GPS receiver to the default configuration, get an appropriate image from u-blox (fitting your receivers serial number), find the flash identification flash.txt file in the u-center install directory and be prepared to wait a long time.
- NOTE: If you have a Tiny with LEA-5H module you must use u-center and manually setup the parameters as shown above (at least switch to 38400 baud manually before transferring the configuration file).
- NOTE: POSUTM is not available on LEA-5H. Instead, use POSLLH. Additionally, add the flag -DGPS_USE_LATLONG in the makefile section of the airframe xml file.
Reset to Default Settings
The GPS module can be reset to its original default settings by pulling BOOT_INT high(3.3V) during a power cycle (Antaris Manual, p. 122). It may be required after a wrong firmware upgrade or a bad configuration change.
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 regions though only WAAS and EGNOS are 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. In 2006 one formerly reliable Paparazzi aircraft began having great GPS problems and displaying very erratic altitude calculations, disabling WAAS immediately resolved the issue and this phenomenon was recreated several times for verification. Turns out a new WAAS satellite was being added to the system and the others were being moved that week for better distribution. Our advice is to completely disable WAAS.
EGNOS issues
EGNOS officially became operational on 1 October 2009. ESA claims that it can determine position to within 2 metres compared with about 20 metres for GPS alone. Note that the service is currently provided only in western Europe. For further information see ESA EGNOS website.
According to the EC the EGNOS transmission will keep the "Do not use" type0/2 flag until it is certified for safety of life operations in (hopfefully) mid 2010. So you will have to enable the "Allow test mode use (Msg 0)" switch in the u-center under UBX-CFG-SBAS together with the correction to try EGNOS.
The u-blox FAQ states the following:
- "Do you see issues with EGNOS?"
- "Yes. The data transmitted by EGNOS in the past were usually good and valuable although sometimes unreliable, e.g. when system tests were performed. u-blox does not guarantee performance and status of EGNOS system. For the latest update about functionality of EGNOS please check the website: European GNSS Supervisory Authority"
Troubleshooting
Problem: I keep getting this error with my nice shiny Tiny v2.1 with a LEA-5H: Invalid_argument("Latlong.of_utm")
Solution: Add the flag -DGPS_USE_LATLONG to your airframe file.
Further Reading
The u-blox System Integration Manual covers a lot of GPS theory as well as product specific topics.
Antenna options for the Tiny and Paparazzi GPS units
The Tiny 1.1 features a 28mm square ground plane intended to be centered below the Sangshin 13mm patch antenna. Much better performance has been seen with the 18mm antennas and an augmented ground plane. The ground plane is a critical part of the antenna affecting not only the gain and polarization characteristics but also the center frequency of the system. Users are advised to expand the ground plane to approximately 36mm square, centered on the ceramic portion of the antenna (not the pin). This can be done with copper foil soldered to the vias of the existing ground plane.
Sangshin 18mm Patch
The Sangshin KSA-ST1580MS18 antenna has proven to offer the best performance of the currently available options. These are available from any Sanshin distributor such as rfmw (here) and cost approximately $6.50/ea. in small quantities.
EMTAC 18mm Patch
Offering identical performance to the Sangshin in a less attractive package is the Emtac 18mm antenna. The part number for the standard 1580MHz 18x18x4mm is ANA1580T18D40 and is not listed on their website. Other frequencies are available on a special order basis and the 1584Mhz has proven to outperform all other frequencies when used with a 36mm ground plane and no radome. The use of a radome (any material covering the antenna) or a larger ground plane should theoretically favor even higher frequencies.
Availability
- TransplantGPS in MN, USA. The 1580Mhz models are usually available at a cost of $3.55ea but there may be a minimum order requirement of ~$50 USD.
Sangshin 13mm Patch
Part of interest: KSA-ST1580MS13
The Tiny 0.99 (not 0.9) and 1.1 were designed around this antenna but users are advised to install 18mm units for better performance.
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
Emtac 20mm Patch
The Tiny 0.9 was designed around this 1583Mhz antenna and performed extremely well. Emtac has replaced this with an 18mm model that they claim offers even better performance.
- Obsolete
Spectrum Control Patch
25mm patch testing on Tiny v2.1. Manufacturer Part Number PA251575008SALF. These are available from Mouser for about $3: 657-PA251575008SALF
Sarantel GeoHelix-P2
This antenna is popular among UAV designers due to it's natural rejection of other radio frequencies such as those originating from the modem or video system as well as it's improved rejection of signals reflected from the ground. U-blox recommends this antenna and features it in their reference design. Frequency and polarization are not dependent upon ground plane geometry so this antenna is sold only in the true GPS frequency of 1575Mhz. The geometry makes this antenna very inconvenient to mount, especially in an airplane. Some very non-scientific testing has been done with one of these antennas connected to a Tiny with a short length of 50 Ohm coax above a 120mm square of ungrounded aluminum foil and performance was adequate. The helical design should theoretically outperform a patch in the air, but not on the ground, so any organized comparison will be difficult. Possibly the most important aspect of this antenna is it's natural RFI filtering, which should be evaluated further.
- GeoHelix-P2 Passive GPS Antenna [datasheet]
Acquiring
To obtain a GPS receiver for use with your Autopilot board, take a look on the how to get hardware page