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SatNav,GPS,Navigation

Tweaking and Testing your GPS Receiver

14th June 2003

 Review by Dave Burrows  

 

Something we find ourselves explaining time and time again in the forums is how to run diagnostic software against your GPS Receiver to check to see if it's working, has a problem, or if you just have it configured wrongly.  These instructions help to walk you through the diagnostic software of choice that we use (Leadtek WinFast Navigator) on the Pocket PC.  The is a version available for the PC also, but this article will specialise in the Pocket PC version.  We will also detail how to test Time To First Fixes (TTFF) with your GPS Receiver which will show you how to reset the receiver into a Factory, Cold, Warm and Hot state.

 

WARNING!  If you do not follow these instructions completely, or you start to mess around with figures in various screens, we cannot be held responsible for rendering your GPS Receiver useless.  Although you cannot physically break your GPS Receiver, if you start messing around with NMEA or SiRF settings you can effectively make your GPS Receiver appear that it's not responding, trying to get it out of this state is difficult to talk you through, so be warned, do not stray from these instructions!

 

Installing WinFast Navigator

We have a version of WinFast Navigator available from our download page.  Download this and install it to your Pocket PC.  Once installed you should find Navigator showing on your Start Menu.

 

Brief Introduction of SiRF and NMEA

Connecting to the GPS Receiver is the first hurdle to overcome, but first a brief introduction.  Firstly there are two GPS Standards, NMEA and SiRF.  Although SiRF is the up and coming new boy on the block, NMEA is the acquired standard and one used by 95% of GPS applications.  Why two standards ?  NMEA is getting old, it was designed to talk at 4800 baud (4kbps).  Modem speeds today are 56kbps so you can see NMEA is slow.  SiRF has brought about that change by increasing speeds and utilising full serial speeds up to 115200kbps.  The thing is although NMEA is slow, some GPS Receivers will work at higher than 4800, but this is what NMEA prefers.  The amount of GPS data sent down the serial port is very small and can easily be transmitted in 4800 baud so although you could reach higher speeds, there isn't really any necessary to do so.  SiRF uses higher speeds because it can transmit the data quickly and put the processor into a wait state which in theory means it utilises less power, newer SiRF chipsets like SiRF IIe also have low power consumption which help even further with sustaining power to the GPS Receiver.

 

Connecting to your GPS Receiver

Now you know there are two standards, and that most applications work in NMEA, what we need to do is connect to the receiver in NMEA.  Run WinFast Navigator and you'll be greeted with the WinFast Navigator Screen.

 

The first thing you need to do is tap VIEW | DIAGNOSTICS.  This will present you with another blank screen.  This is the diagnostics screen and it's best to be sitting on this when you try to connect. 

 

 

If you don't see any legible data appear on screen when connected (later), then you've got the wrong settings.  Connecting to the GPS Receiver is hit and miss, trial and error until you find the correct settings.

 

Now tap TOOLS | PORT SETTINGS and you'll be greeted by a screen like below.  This shows you the protocols available (eg NMEA and SiRF), the Serial Port you will connect on and the Baud Rate to use. 

 

We said earlier we would use NMEA, so make sure this is selected, skipping to the baud rate, we know that NMEA prefers most of the time to use 4800 so we can set this easily. 

 

Jumping back to Port, if you know you are connecting to a GPS Receiver that plugs into the bottom of the sync port, 99% of the time this is regarded as Serial1 COM1, so you can set it to this. 

 

If however you have a Compact Flash GPS Receiver, the easiest way is to cancel, eject the receiver, and then plug it back in again and it should pop up a dialog box and tell you what COM port it will use. 

 

If you're using a Bluetooth GPS receiver, you can check the COM port that's being used by going into the Bluetooth Manager and checking the settings as it should tell you which COM port will be used for Bluetooth connectivity.  With that said, we should now have all of the settings we need.

 

 

Tap the OK button.  What should happen is that you'll be taken back the the Development screen in WinFast Navigator.  You should see a screen like the one below (left).  If you don't, and you are seeing a screen like the one below (right) then you have the wrong settings.

 

 

The data in the above (left) screenshot should start appearing within 5 seconds of reaching this screen.  If it doesn't, go through the process again.  If you are sure you have the correct COM port, then all you need to do is change the baud rate.  Try 9600 (this is what the Pretec Compact GPS Receiver uses).  Other receivers like the Pretec LP use COM 5 or COM 6, the Navman sleeve does something similar.  Like we said above, it really is a matter of trial and error.  At this stage you can not break the receiver, all you are trying to do is connect to the device on the above settings, so don't worry if it doesn't work, it won't cause problems with other applications as you are not saving anything to the EPROM (chip).

 

At this stage if you have managed to successfully see the data coming through on the development screen and you have been experiencing problems with your GPS Receiver not working, then there is nothing wrong with the receiver, the problem is probably in the driver of the software you are using.  If you were experiencing problems and still are, with no data appearing on screen, then it could be either you still have the wrong settings, or the GPS Receiver is faulty.

 

Testing for Time To First Fixes

We know that TTFF's can change, and do erratically.  It all depends on whether you have clear line of sight to each satellite, and there are countless things that can delay the data, or the GPS program assumes data which may not be correct.

 

Once you have a successful connection and you have NMEA data coming through on the Development screen, then what you can do is tap TOOLS | COMMAND and you'll be greeted by a screen like to the right.  This will by default put you on the Initialize tab.  In here you have options for resetting the GPS.  If you didn't know, the GPS has two types of databases.  The Almanac details what satellites are out there, and their projected positions in the sky, and this is usually current for a couple of months use.  The Ephemeris data is more accurate data about signal strength, altitude and where they are exactly in the sky.  The Ephemeris data is only current really up to a couple of hours, after this time you have to download the complete Ephemeris data which takes 30 seconds under supreme conditions.  Almanac data with Ephemeris will take around 12.5 minutes.  One way of testing a receiver in a cold state would be to acquire a 3D fix and then power it off for 4 hours, then test again and you should then have to re-download the Ephemeris data.  Another way is to foil the GPS into thinking it doesn't have it by wiping that data.  This is what the options on this screen does.  A Cold restart will lose all Ephemeris data, a Warm restart will lose a small segment of this data, and a hot start will lose virtually no data, and this will effectively simulate a GPS Receiver being powered off for a period of time.  Once you click Hot, Warm, Cold or Factory and leave the MESSAGES tick boxes enabled and tap OK, this is what you will be doing, simulating this type of start.  This is a harmless test, and you can time TTFF's yourself.  The one thing to bear in mind is you can initiate a cold start and it will come in say at 49 seconds, another cold start may come in at 36 seconds, a third cold start will come in at 53 secs.  You'll see a picture forming here, no TTFF will ever be the same due to conditions and data.  That's why in our tests we take 5 Cold TTFF's and take an average.  Cold TTFF's are what people are mainly concerned about, mainly because a Cold TTFF is what you'll see when you come home from work, power off the GPS Receiver, then power it on 8-12 hours later in the next day when you go to leave for work.  Cold TTFF's are always what people complain about if it takes a long period of time.  Now if you are receiving a TTFF of 2 mins plus, you either don't have a good sight of the sky, or you have a weak antenna, if you see TTFF's of 3-5 mins, get a new receiver, it shouldn't take this long!  50 seconds is probably the norm you will see, certainly below 60 seconds.  Remember when you tap OK you'll be taken back to the Development screen.  Once you're happy you know data is coming through, you can select VIEW | SIGNAL LEVEL or VIEW | NAVIGATION for a different perspective of where satellites are.

 

Getting Technical

Okay, so you've been messing about a bit and you feel as though you're happy with all of the above, you can consistently get a connection with the GPS Receiver, and you're happy with it's output.  You want to start tweaking the GPS Receiver.  Perhaps with the GPS Receiver you are using, you're getting the amount of satellites you have a lock on, but you're not seeing any signal strength in the GPS application you're using, or in WinFast Navigator ?  This is where we get technical.  Under NMEA (protocol) you get different string types.  All of these contain data like a database.  A table of the current NMEA sentence codes are listed below and a description has been added.  We have also highlighted some sentence codes in bold.  These are the ones you will normally see a lot of.

 

String Type Description
$GPAAM Waypoint Arrival Alarm
$GPALM GPS Almanac Data
$GPAPA Autopilot format "A"
$GPAPB Autopilot format "B"
$GPASD Autopilot System Data
$GPBEC Bearing & Distance to Waypoint, Dead Reckoning
$GPBOD Bearing, Origin to Destination
$GPBWC Bearing & Distance to Waypoint, Great Circle
$GPBWR Bearing & Distance to Waypoint, Rhumb Line
$GPBWW Bearing, Waypoint to Waypoint
$GPDBT Depth Below Transducer
$GPDCN Decca Position
$GPDPT Depth
$GPFSI Frequency Set Information
$GPGGA Global Positioning System Fix Data (Time, Position, Elevation)
$GPGLC Geographic Position, Loran-C
$GPGLL Geographic Position, Latitude/Longitude
$GPGRS GPS Range Residuals
$GPGSA GPS DoP (dilution of precision) and Active Satellites
$GPGST GPS Pseudorange Noise Statistics
$GPGSV GPS Satellites in View
$GPGXA TRANSIT Position
$GPHDG Heading, Deviation & Variation
$GPHDT Heading, True
$GPHSC Heading Steering Command
$GPLCD Loran-C Signal Data
$GPMSK Control for a Beacon Receiver
$GPMSS Beacon Receiver Status
$GPMTA Air Temperature (to be phased out)
$GPMTW Water Temperature
$GPMWD Wind Direction
$GPMWV Wind Speed and Angle
$GPOLN Omega Lane Numbers
$GPOSD Own Ship Data
$GPR00 Waypoint active route (not standard)
$GPRMA Recommended Minimum Specific Loran-C Data
$GPRMB Recommended Minimum Navigation Information
$GPRMC Recommended Minimum Specific GNSS GPS/TRANSIT Data (time, position, velocity)
$GPROT Rate of Turn
$GPRPM Revolutions
$GPRSA Rudder Sensor Angle
$GPRSD RADAR System Data
$GPRTE Routes
$GPSFI Scanning Frequency Information
$GPSTN Multiple Data ID
$GPTRF Transit Fix Data
$GPTTM Tracked Target Message
$GPVBW Dual Ground/Water Speed
$GPVDR Set and Drift
$GPVHW Water Speed and Heading
$GPVLW Distance Traveled through the Water
$GPVPW Speed, Measured Parallel to Wind
$GPVTG Track Made Good and Ground Speed ( Course over ground and ground speed )
$GPWCV Waypoint Closure Velocity
$GPWNC Distance, Waypoint to Waypoint
$GPWPL Waypoint Location
$GPXDR Transducer Measurements
$GPXTE Cross-Track Error, Measured
$GPXTR Cross-Track Error, Dead Reckoning
$GPZDA UTC Date / Time and Local Time Zone Offset
$GPZFO UTC & Time from Origin Waypoint
$GPZTG UTC & Time to Destination Waypoint

 

What does an NMEA and SiRF stream look like ? 

Take a look at the two screenshots below, the first is an NMEA stream, and the second is a SiRF stream.  As you can see, these are totally different and require specific knowledge on both NMEA and SiRF string sets.

 

 

Now one problem a lot of people experience is that you are receiving the total number of satellites you have a 3D fix on, but you are not seeing any satellite information for each satellite that would usually show signal strength.  The reason for this would be that GPGSV isn't enabled as part of the NMEA string.  You'll see this in the first screenshot below.  If you exit TomTom GPS Status screen and then run up WinFast Navigator, connect to the GPS Receiver, then go into the COMMAND option and for this test we will make sure GSV is set at 5, then tap OK.  What I would recommend is that you set GGA=1, GLL=0, GSA=5, GSV=5, RMC=1, VTG=1, MSS=0, User=0, User=0, User=0.  When you tap the OK button, it should then enable it.  Disconnect, run up your regular GPS program and check the satellite status now, you should now see satellite bars like in the second screenshot below.

 

 

Later on you may want to change the above settings, e.g. these will depend on the requirements you have against your navigation program.  Do you see the need to see the satellite status every five seconds ?  Or is it enough to see them every 20 seconds ?  All you really need for your navigtation program is a frequent update of your location (either RMC or GGA will do) and speed (VTG).  Again when you make these changes verify that your navigation program does still work as expected.  Unfortunately some applications prefer different sets of base data, so you may have to adjust the NMEA output to your program's preferences.  You also have to be careful when changing these settings on the NMEA and SIRF screens.  The settings you see here are not from the GPS Receiver, these are default settings within WinFast Navigator. 

 

There is no way of easily telling what the GPS is set to with WinFast Navigator, so if you set something here, it could look like it's rendered your GPS Receiver useless.  If you start getting garbage appear on the DEVELOPMENT screen, e.g. lots of % or fraction symbols or square blocks, then there's something wrong. 

 

This can sometimes happen when you try to set the GPS to an invalid baud rate.  Simply switching it back to the correct baud rate can still appear as though the receiver is not functioning correctly.  You'll see some garbage in the screenshot to the right.

 

The best thing you can do here is to switch it to SiRF mode, you will see a different set of text (eg not the sentence strings in the table above), but you should be able to read it as the text will be in English.  If this then works, go back into the COMMAND option and switch back to NMEA.

 

Breakdown of the GPS Sentence

If you take the GPRMC line from the NMEA sentence as:-

$GPRMC,181573,A,4823.029183,N,07384.874834,W,000.09,183.4, 131200,1.8,W,D*25

You will then be able to break this down as follows:-

$GPRMC = Sentence ID


181573.00 = UTC (Coordinated Universal Time) of position fix in hhmmss.ss format (18 hours, 15 minutes and 73.00 seconds)


A = Status (A = data is valid, V = warning)


4823.029183 = Geographic latitude in ddmm.mmmmmm format (48 degrees and 23.029183 minutes)


N = Direction of latitude (N = North, S = South)


07384.874834 = Geographic longitude in dddmm.mmmmmm format (73 degrees and 84.874834 minutes)


W = Direction of longitude (E = East, W = West)


000.09 = Speed over ground in knots (0.04 knots)


183.4 = Track made good (heading) (183.4)


131200 = Date in ddmmyy format (December 13, 2000)


1.8 = Magnetic variation in degrees


W = Direction of magnetic variation (E = East, W = West)


D = Mode indication (A = autonomous, D = differential, N = data not valid)


25 = Checksum (following asterisk delimiter)

If the GPS receiver is not operating properly or it hasn't received a 3D Positional fix then most of the values will be left out and you will just see commas instead.  In the above example you can see that both the latitude and longitude values included in the NMEA sentence are represented in degrees, minutes, and decimal minutes. The latitude is shown as ddmm.mmmm, and the longitude is shown as dddmm.mmmm (see the extra d). You will then see the direction of Latitude and Longitude as N, S, E, W (North, south, east and west).  

 

Summary

Hopefully this tutorial has given some insight into what happens behind the scenes with your GPS Receiver, and has helped you see how GPS operates and how to fault find problems with your GPS Receiver.  Hopefully it has also empowered you into taking your own TTFF's and seeing whether or not you should be looking to change and upgrade to a better GPS Receiver.

 

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