GPS Receiver Technical Note
SiRF-Based GPS MODULES M10214 and derivatives
Ephemeris Push
1. Background
When a GPS receiver makes a cold start,before it can abtain a position fix it needs to synchronise the signals from several satellites – a minimum of 4 for a 3-D fix- and also to receive precision information about the orbital position of each of these satellites (SV s).The orbital positions are fixed with a precision better than 2m,which is obviously time consuming when the data rate of the link from the SAV is only 50b/s- limited by the very low received signal level.
Unless some additional information is provided, the cold start process takes typically 40s under open sky conditions. A warm start is often no quicker.
We can speed up a warm start by giving the receiver the following information.How this is used is described later.
1. An approximate current position(within say 1000km- in other words the last known position will usually do fine.)
2. The approximate time. This can be from your watch, a clock on the platform,GSM network time…anything.
3. A valid set of ephemeris data(ephemeredes) relating to SVs currently visible to the receiver.
Individual SVs are visible to a receiver for 4 hours.At a given time,some will be just rising.others will be in the middle of the period of visibility,while others will be just setting,so the average period for which a set of SV data will be useful is 2 hours.If we use the SiRF prococol action Poll Ephemeris the complete set of ephemeredes for currently visible SVs is saved to a file with the default name a phemris.eph.Typical contents of this file are shown in Appendix 1.
2 The Ephemeris File
The file begins with a time and date stamp. It then contains an sequence of data items for each SV with has been in view for long enough and with a sufficiently high C/No to allow valid ephemeris data to be obtained. SVs not in sight are represented by entries filled with zeros.The file is in text format and has a typical length of 7k bytes (for memory allocation purposes the maximum length is probably less than 9kb).
An ephemeris files (E-file) as a whole is valid WHILE the time stamp is less than 4 hours before the time when it is passed to the receiver. If the file itself has a valid time stamp the data on individual SVs is examined; some may be rejected as no longer valid.
The objective of passing the E-file to the receiver is to allow it,once it has established current GPS time from the first SV reaching 3F synchronization status, to bypass the need to download ephemeris data for each separate SV and to be able to create a rapid fix. To do this, at least one SV must have a C/No that exceeds 28dB. If 3 SVs reach 3F,then on accepting the E-file, a fix is almost instantaneous. All SVs listed in the pushed E-file and currently visible will be locked (green) simultaneously, including those with current C/No lower than 28 Db.
3 Demonstrating E-push
To demonstrate E-push a module must be flashed with FW version:
GSWL T3.2.5Antenova_3.3.01.06-CL31P1.00
As well as supporting E-push, this is version has the following defaults:
NMEA protocol
9.6kb/s
Default to sleep after starting
This is currently a Beta version,suitable for demonstration and prototype use.
To support E-push,select SiRF-Binary (S-B) mode you must. tick the Enable NavLib Data.
Also note that to obtain outpout in the Debug window while in S-B mode you must,tick the Enable Debug Data checkbox as well
As noted above, you also need the current time (which needs to be in GPS format:GPSWeekNo,seconds) and a rough estimate of current position (Lat,Long,Altitude).This data is transmitted to the receiver using an NMEA command.
3.1 Doing it
1. Set up your module on an Eval Board and start is; leave it to gather as many SVs as possible.
2. Run SiRFDemo,turn on Navigation View and write down the Lat,Long,Alt,GPSWeek and Time. (You can’t copy from this window.)
3. Open a TXT file and create a line with content modeled on the following:
PSRF104, 13.13501, -61.20218,8,96000,341725M431, 12,3
This has the following content:
PRSF104 The NMEA message identifier.
13.13501 Present latitude (use your present or last one available)
-61.20218 Present longitude (W is –ve, E is +ve, use last available)
8 Approximate altitude (m)
9600 Default place holder
341725 Approx current time (in GPS seconds)
432 GPS week (modulo 1024)
12 No of SVs (always use 12)
3 Specifies a Hot Start with Inits
The reason for doing this in a separate file is that you can save it for reference next time!
4. Select S-B mode
5. Select Poll/Ephemeris
6. Save E-File (use the default name and put it on your desktop).A few seconds later the program will confirm it has been saved.
7. Select NMEA mode
8. Select Action/TransmitSerialMessage
9. Paste the time from your text file into the window (Now you know why you created a text file!)
10. Click the Radio Button Protocol Wrapper NMEA
11. Click Send. In the signal view you will see that all SV lines immediately turn red.
You have performed a restart. Now, rapidly;
12. Change to S-B protocol
13. Select Set Ephemeris and WAIT with the selector window open until at least one SV status is shown as 3F
14. THEN click the button to sent the E-file.
15. Most SVs will quickly turn green and a fix will be shown.(If 3SVs were at 3F this will be almost instantaneous in my experience).
YES,this looks complicated,and to some extent it is. Next time your try, all you need to do is to update the text file with current time and week (but only update the position if you’ve been flying since you last used it).
It is not easy to time the fix(TTFF) if you are doing this by yourself.The TTFF shold be measured from the time the NMEA reset message is sent, but you are busy changing protocols, Setting Ephemeris and waiting for 3Fs. For this reason the time seems shorter to you than to an objective observer.
To repeat the operation you just need to go back to step &The message will still be in the window unless you have terminated and re-started SiFFDemo, in which case you can recover it from your TXT file and update it. All the check buttons and radio button selections stay as defaults unless you re-install SiRFDemo.
3 Programming all this
Fortunately doing this under program control is rather easier than doing it by hand.
Poll Ephemeris is executed after receipt of S-B message ID147, DESCRIBED ON PAGE 2-18 of the S-B manual. The example in the manual is to get the data for all satellites, which is what is needed (because you don’t know a priori which are to be seen).
The return message is ID 15 (page 3-22).The formate of the response is specified in the detailed description of ID15.
There are no corresponding NMEA instructions for this.
The reset message is NMEA $PRSF104… see the SiRF NMEA manual.Programming it will require recovery of previous position data and the time.
The SetEphemeris message is S-B message ID149.
Although this description hides some of the complexities of format, this is essentially a simple 3-step operation. Protocol switching is clearly described in the SiRF NMEA and S-b manuals
Please report problems with implementation and observations about the effectiveness of E-push to the author of this note, Brian Collins.
