DGPS Ground Station?

[risto]

Hi,

I want to build a automated flying model helicopter. Might be a crazy idea, but I still want to do it.

My actual problem point is controlling the model precisely.
Currently I have played with GPS (you can take a look at: http://www.mh.ttu.ee/risto/rc/logo20/wvideo/ ).

Standard GPS is not very precise (the position changes even if the receiver does not move...), so I have looked into DGPS. I do have a DGPS receiver (just like a standard GPS, just with one extra serial port for inputting Satellite correction data). I also have a radio modem for sending the data to the DGPS receiver from some fixed base station nearby.
The only thing I do not have is a DGPS ground station itself. After searching for a week in Internet and sending many emails to companies, I seem to find no company that makes/sells such thing. Maybe anybody in this Forum can help me out?

Many thanks,
Risto
http://www.mh.ttu.ee/risto/rc/models/

[Petri Nygren]

Tere Risto!

You have certainly taken a big project this time.

I would recommend you to search with words "RDS DGPS". In most European countries, there is a service based on sending real time positioning corrections via FM radio stations as one of the normal RDS messages like local traffic instructions etc. A lot of European countries have adopted RDS DGPS technology by US company "Aztek" (not "Ashtech"). Unfortunately, I do not think Germany is one of them because they want to do it better :rolleyes: and are thus uncompatible.

Code-differential RDS corrections will get you down to about 1-10m accuracy dynamically. The system is out of favour now since DoD quit degrading their GPS service intentionally (thing called selective availability), so everybody is getting a position highly satisfactory for the mainstream of navigational users. Thats what you are seeing now and it´s somewhere around 30m.

A service called Real time Kinematic (RTK) is also sent via RDS at some areas. That will give you geodetic accuracies better than 30cm but unfortunately equipment is professional only and priced accordingly. Not to mention that both code and RTK RDS DGPS are services for a fee.

Option number two for you would be what you suggest, i.e. building a DGPS system of your own. You would normally need two similar GPS receivers for that, equipped with differential mode. Other one, the Base station is at known location and the other one, the Remote, is in your chopper. Base is setup to send code or phase corrections via radiomodem, and Remote accordingly to receive them. If your receiver has no diff capability, or if the receivers are of different brand, its going to be hard or near impossible to build a real-time DGPS network.

Third option would be to get corrections via an Inmarsat satellite service like ones provided by companies Fugro or Racal. But the satellite radiomodems they require are bulky.

Net is blistering with equipment producers. Check these for example:

http://www.ashtech.com/en/
http://www.trimble.com/
http://www.topconeurope.com/surveying/gps/indexgps.html
http://www.novatel.com/
http://www.gpsworld.com/gpsworld/


Hope that helps a little.

Petri

[Choppa Nutta]

I like this idea, had a similar idea for indoor stuff using 3 base stations transmitters and a receiver that could triangulate its position in 3D space, though conjuring the electronics might be another matter :) would work on the flat and with other sensors and a map in its head of the terrain might allow some really precise navigation, but range would probably an issue, sounds like you want to be able to cross the channel and back :D


dunno if you have seen these similar and other ideas--

http://web.mit.edu/whall/www/heli/

http://www-2.cs.cmu.edu/afs/cs/project/chopper/www/heli_project.html

or read this ?

http://www.rcgroups.com/forums/showthread.php?s=&threadid=17855&highlight=autonomous+helicopter

just for interest :)

http://www.rcgroups.com/forums/showthread.php?s=&threadid=56999&highlight=autonomous+heli

[risto]

Moi Petri,

First many thanks for the links and for your suggestions.

I think your first suggestion (RDS DGPS not in Germany; RTK) are unfortunately not so good for me as they are unavailable or cost more than getting a DGPS station.
Self building a DGPS base station requires special information that cannot be read out from normal GPS receiver (you need for each satellite separately the information - a normal receiver outputs all summed up). The theory found under http://www.trimble.com/ says:
"Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the reference receiver quickly runs through all the visible satellites and computes each of their errors."
Read more: http://www.trimble.com/gps/index.html
Basically meaning, one should build the whole GPS receiver oneself. Huh... that's a bit too much for me.

But under the links, I found many useful things:
Under http://www.ashtech.com/en/
Micro-Z CGRS, that seems to be able to do just what I need.

Under http://www.topcongps.com/ found also:
their HiPer station.

Choppa Nutta,
thanks for the links.
Here's more one good one:
http://user.cs.tu-berlin.de/~remuss/marvin.html

What I am trying to achieve:
1) a heli that would just hold its position regardless of wind etc.
2) if that works, then a heli that would fly to a certain point (x,y,z) and stay there
3) if that works, then a heli that would hold itself in certain direction in certain point (x,y,z,d)
4) if that works, then a heli that would fly a predetermined route
5) if that works, then a heli that automatically avoids collisions when it flies the route
It's lot of ambition, but it can be done. And it can be done pretty fast with lots of $$$, but I trying to build it cheapest possible way, but in no way still sacrificing in quality. That's the difficult point here.

Risto
http://www.mh.ttu.ee/risto/rc/

[Petri Nygren]

Originally posted by risto
"Since the reference receiver has no way of knowing which of the many available satellites a roving receiver might be using to calculate its position, the reference receiver quickly runs through all the visible satellites and computes each of their errors."
Read more: http://www.trimble.com/gps/index.html
Basically meaning, one should build the whole GPS receiver oneself. Huh... that's a bit too much for me.



Risto,

It´s not that hard really, because the difficult part is standardised for you already. There´s a standard called RTCM SC-104 which is a transmission format always containing correction information for all the satellites in wiew. It can be setup to include code (1-10m) - or RTK (1-30cm) measurements. So you just have to locate the cheapest receiver you can which can be setup to transmit/receive RTCM SC-104 messages via it´s RS-232 port. For example, the market leader Garmin seems to have equipped most of it´s models with RTCM SC-104 capability.

If I had the money, I would go for Ashtech Z-Eurocard:
http://www.ashtech.com/assets/datasheets/ZEurocardBoardDS.pdf

It weighs just 240g and is pocket-size geodetic receiver, with 12-channel, L1/L2 dual frequency, C/A code and RTK capabilities. Common receivers are 1-3 channel, L1, C/A code receivers only.

However, that would still require a separate, bulky radiomodem. By far the best solution would be to get two of the new GPS-equipped nav-mobile phones. That would keep receiver, radiomodem, antennas and even their lithium power source within the same, light unit. Simply put one to your chopper and another to a location with known coordinates.

http://www.benefon.com/products/index.htm
http://www.garmin.com/products/navTalkGSM/
http://www.nokia.com/phones/gps_module/index.html

There´s even one upcoming walkie-talkie -kind of model for up to 8km ranges:
http://www.garmin.com/products/rino/


Regards,

Petri

[risto]

Hello Petri,

thanks again for lots of information.
> If I had the money, I would go for Ashtech Z-Eurocard:
> http://www.ashtech.com/assets/datas...cardBoardDS.pdf
I take a look how much do they require for that - with educational price :)

> However, that would still require a separate, bulky
> radiomodem.
That's not so big problem. I have Siemens M101 Data radiomodem set, it goes up to 115,200bps with up to 300m range with standard antennas, that is enough for my heli at first.
http://www.my-siemens.com/com.aperto/MySiemens/Images/Product/ho/tl/pcc/gigasetm101data_1.jpg

Risto

[Corky Boyd]

You don't need to have your own differential ground station. As in the US, european governments transmit the correction signals. You need a dgps reciever and a list of frequencies. The most common are in coastal areas and the signals provide excellent corrections within 300 km of the stations.

You can purchase commercial differential recievers from marine electronics stores. In the US, the latest differntial system is WAAS which transmits correction signals directly from the sattelite so only that a single reciever is needed. I believe a similar system is being used in europe under the name of Fieldstar.

From my boating experience, I find that the altitude accuracy, using differential, is within 3 meters about 95% of the time. Altitude is the only dimension I know with certainty. By mathematical inference, latitude and longitude would be within 2 meters 95% of the time.

Check with your local marine electronics stores for what is available in your area.