OK this is a question for anyone who knows more about DGPS than I do and about what RDS data we can access over the soon to be released tuner for the MkII...

I have acquired a GPS (DeLorme GPSTripmate) that I want to connect up to my empeg to start playing with the idea of implementing some basic navigation stuff to display on the empeg's display directly (probably using some form of the Clock kernel hack to display location, speed, distance and direction to waypoint overlaid on the normal player display)

Being new to GPS I have discovered this thing called DGPS to increase the accuracy of GPS data with updates from a DGPS server... and have found out there is such a DGPS server in my area that broadcasts using RDS...

Would it be possible to get this data through the new tuner module that supposedly can receive RDS...? Does anyone out there know if the model of GPS I have can make use of it...? can it be used in software...? does anyone know of a good piece of software to use on the empeg...? I am currently looking at gpsd as it seems to do what I want... but haven't got to the point of trying to build it for the empeg...

Any pointers would be great... :)

Cheers

Kim

Kim,

Basic DGPS is a system that calcuates the error (induced or other) in the received satellite signals and then broadcasts a correction code that can be used as input in a regular GPS to modify the received signals for better accuraccy.

A GPS that is DPGS compatible will have a serial input that allows the DGPS receiver (or RDS receiver) to send the serial correction information it. I don't believe that the low-cost 1st gen unit is compatible (a quick check of the Delorme site shows no DGPS capability).

Certainly the RDS data stream which is outputed by the tuner module and (the appropriate) information displayed on the empeg's screen could certainly be used to create a serial data stream for the DGPS capable unit.

(gotta run - plane just closed it's door!)



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Queue 351, Now Mk II #60000022 18gb, Blue

OK thanks for that...

Is it possible to do the correction in software...? ie take the dgps info from the RDS stream and the input from the GPS and then do the adjustment in the software reading from the GPS...? Or is it something that is required in the GPS internal calculations...?

The Tripmate is just a receiver with a serial cable... I have found some links to some command strings you can send it... but I think they are mostly to do with changing the format it outputs... some of the programs I have been using with it have the ability to use DGPS over IP... but maybe that info is just being ignored with my GPS if it isn't able to use it...

Cheers

Kim

Hey, I had the same idea, and I have not gotten much farther yet. gpsd seems to lack a good sample command line program to retrieve data and I was not up to wading through the GUI sample. I started to look at a package called jeeps that seems a little better. It compiles and runs OK on the empeg, but has one bug. Long and lat readings don't work. I get large negative numbers or 0. Elevation, heading, etc seem to work fine. I am guessing that it is some sort of endianness issue, but have not had a chance to fight with it yet. Aside from that the API seems really easy to deal with. Anyway, let us know what you end up doing. And let me know what you end up using to talk to your GPS.

-Mike

In reply to:

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Is it possible to do the correction in software...? ie take the dgps info from the RDS stream and the input from the GPS and then do the adjustment in the software reading from the GPS...? Or is it something that is required in the GPS internal calculations

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My understanding of the correction signals is that they contain several pieces of information: for each satellite a DGPS beacon message containes the satellite number and the correction factor (in nano seconds) to be applied to that particular satellite.

That modifier is used to apply a correction to the internally calculated transmission times from each satellite - which is then used to figure out your location.

It's not as simple as 'satellite A is reporting a position that's 43M North NE of where it should be' so it's not a gross modifier that you could apply to an already calculated position.

To end up on an up note. You can buy a Garmin LP35 receiver that's DGPS capable for less than $200. Spiffy unit (without display) that's housed in the antenna - all you have is power and serial leads. If you can get the correction messages from some kind of beacon or another (RDS, Internet or radio) then you can input that directly to the LP35 for it to output 1.5M accurate information.

=-C


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Queue 351, Now Mk II #60000022 18gb, Blue

Kim,
now that the US miltary has switched off the SA feature in the GPS system, you will find that the current level fo accuracy is pretty good for navigation.
DGPS can be used to increase this accuracy - usually the DGPS signal which is in a format called RTC104 [from my very rusty] memory. This has the 'corrective factors' to be applied to the information supplied to your GPS receiver by each the GPS satellites that are currently in view and being used to determine the GPS receivers location.

This corrective factor (or factors - at least 1 per GPS satellite) is then used to adjust the actual calculated lat/long/altitude the GPS receiver displays/outputs.

Most GPS receivers that have a DGPS input usally need to be given the corrective information in real-time via their DGPS input which is like a serial port [at TTL level].

The other point is that although the local Radio Station may output DGPS information on RDS, is the DGPS information transmitted in the clear [no encryption] or scrambled [with some encryption of the data].
If the latter then the DGPS information will have to be decrypted first.

In the old days [well say this time last year],in the US, DGPS broadcasts via RDS were routinely encrypted and only available to subscribers for a small annual fee [plus the right equipment].

Whether this is still the case I do not know. However more than one company in the US makes aliving from selling DGPS information over a RDS stream from a local FM station.

This place contains lots of details about the DGPS system including info on hardware hacks and source code for an internet based corrective system.

Talk about needing a long extension cable--perhaps if you have an internet hookup in your car...

My playing with gpsd so far has entailed starting the daemon with the -D flag so it outputs debugging information and then telneting to localhost port 2947 and hitting R to force it to start outputting data...

as for the lat long readings my guess without having played with it on the empeg yet is that this might be an issue with the fact the StrongArm doesn't use floats...?

I am hoping to get a chance to spend some time getting it running on the empeg next week when I have a week away from the office...

Kim

Yes... this is why I am not incredibly concerned about getting DGPS working... but it seemed that since the new tuner module can deal with RDS it might be something to look into and would be cool if I could get it working... :)

I don't know whether the local DGPS over RDS stream is encrypted yet... it is run along side the frequency of JJJ here which is the Australian Government broadcaster (ABC) "youth" radio... therefore I have a hope it is free service...

Kim

*nod*

I had been to this page and it is one of the ones that made me think that the DGPS info might be able to be dealt with from the software side... but reading further I see that GPS data can only be post-processed with DGPS if you can get the receiver to output pseudo-range information...

So I have to work out whether I can get my receiver to output this pseudo-range data (unlikely) or find out whether my receiver has any commands to upload DGPS data to it (probably just as unlikely)

I have found a few pages that list some of the commands people have got working with the Tripmate... seems most have been found by trial and error so maybe if I could work out what commands are used to upload DGPS data to other GPS receivers one of those might work...

I will have a hunt around the gpsd sources...

Kim

if you use gpsd in raw mode it's beaviour is just like the direct serial connection to the gps receiver.
and NMEA-parsing is quite simple....

bobo