RASANT DGPS Signals in Europe bring low cost DGPS to many GM100/G-III  European users
By Carlos PUCH/Madrid Spain

RASANT is a real-time DGPS system which is broadcast on a FM sub-carrier by FM radio stations around Europe using the FM Radio-Data-System (RDS). The transmitted corrections must be decoded to RTCM format by a special receiver, such as the ERTEC RDS66 (see: http://www.ertec.com/).

Some testing with the unit has proven a couple of interesting facts: The RDS66C (C for *compact*) sends corrections which had now demonstrated that these signals are received and acknowledged by the GM-100 and by the Garmin G-III as well.

On the flat roof of our building, with good geometry and 8 satellites in view, I have seen a consistent EPE of 1 meter during the 25 minute test.

The GM-100 must be programmed so that:
1. The baud rate is adjusted to 9600, like the RDS66C output.
2. The Starlink mode (or STAR in DGPS config menu) is ON.

In group G window all satellites in view are listed on the upper part of the screen. A "OK" sign on the right of each corrected satellite is shown. On the lower screen only the bit rate and the age of the corrections are shown. The
rest of the fields, which are designed specifically for the Lowrance DGPS Beacon Receiver, remain blank.

When I got  to retesting the G-III interface to the RDS66C output,  I discovered that the data cable which I was using (a Garmin original, by the way) was defective: it only worked one way (G-III -> PC),  but not the opposite. After replacement of the sub-D 9 connector, Bingo !: a *D* appeared on each bar of the satellites page. As with the GM-100, the Main Menu page "INTERFACE" didn't show any other evidence of the arrival of the differential corrections, but they were there for sure (very low EPE and DOP)...

The end of this long (and winding) story is that both,  the Garmin and the Lowrance receivers, accept the RTCM
output of the Ertec RASANT receiver.

For we all, european users, it's a good news that we can have DGPS corrections for free (well, a $17.00 fee is included in the price of the receiver, but you only have to pay it once...). The corrections are widely available,  i.e., in Spain all the FM stations of Radio Nacional (the public radio network) transmit RDS, and most of them send RASANT corrections. So that, in theory, they're available everywhere. I have to test the system on the back country... I'll report to the Newsgroup.

For memory, the spanish reference stations are controlled by  IGN (National Geographic Institute).

The price of the RASANT receiver (including software for programming the unit -runs on W95/98-, cables and manual) is $380.00. The GM-100 power/data cable needs a 9/9 pins gender changer to be connected to the unit.


Carlos Puch (Madrid, Spain)