Hacked DGPS Beacon Rcvr?
Richard Cochran
I'm doing some volunteer mapping work for habitat restoration where I
need approximately 10 meter accuracy on a budget. I already have a
Garmin GPS45, which could probably do the trick using the USCG DGPS.
I also have a portable shortwave radio that can easily receive the
signal from a nearby USCG Beacon, but of course it can't decode the
data.
My question is this: Can I use a RTTY/ASCII/CW decoding box, built for
Ham and Shortwave use, to convert the DGPS beacon to digital and feed
the data into the GPS? I was thinking of using the MFJ-1225
interface, which I think ought to do the trick, at only $70.00 US, far
cheaper than the $500.00 needed for Garmin's DGPS beacon receiver.
I know that if I go this route, I'll end up with a clumsy contraption
of shortwave, decoder, GPS, and a big battery to power the mess, but
I'll be able to fit everything into a backpack. I'm not sure if the
decoder will be able to decode the Coast Guard's MSK modulation
format, or if I'm missing some other crucial detail.
Anybody know of a good reason why this will or won't work?
Unless somebody knows it won't work, I'll probably order the
interface, try it out, and post results. The worst thing that can
happen is I will have blown $70.00 on a decoder that I've kind of
wanted to have for a long time for other purposes anyway.
--Rich
David Wells
In article <rcochranD...@netcom.com>,
Don't spend the money. The MFJ etc. will not work since you need a demodulator
that specifically works with MSK. Also, you will need to convert the received
data to RTCM SC104 format to feed it to the Garmin.
Dave
Bill Pechey
I think it'll work but you may need to invert the data and you will also
need a good signal strength from the DGPS beacon.
Go for it and ley us all know how you get on.
Bill.
Barbara Fehner
I am getting the MSK signal from the Pt. Loma beacon using a AEA
PK-232. It seems to handle the 25 Hz FSK shift ok although the BER is
probably higher then normal. My question is what to do with the bit
stream now. What does the bit stream use for synchronization? Does it
use start and stop bits? Is the stream encoded as ASCII or is it
straight binary. The coast Guard WWW site says that it conforms to
RTCM SC104 but I can not find any other info on the net. Does anyone
have a copy of RTCM SC104 or the Coast Guard Broadcast Standards paper
that can clear up these questions?
rcoc...@netcom.com (Richard Cochran) wrote:
>I'm doing some volunteer mapping work for habitat restoration where I
>need approximately 10 meter accuracy on a budget. I already have a
>Garmin GPS45, which could probably do the trick using the USCG DGPS.
>I also have a portable shortwave radio that can easily receive the
>signal from a nearby USCG Beacon, but of course it can't decode the
>data.
>
>My question is this: Can I use a RTTY/ASCII/CW decoding box, built for
>Ham and Shortwave use, to convert the DGPS beacon to digital and feed
>the data into the GPS? I was thinking of using the MFJ-1225
>interface, which I think ought to do the trick, at only $70.00 US, far
>cheaper than the $500.00 needed for Garmin's DGPS beacon receiver.
>
SNIP
Judson L. Ahern
Barbara Fehner wrote:
>
> I am getting the MSK signal from the Pt. Loma beacon using a AEA
> PK-232. It seems to handle the 25 Hz FSK shift ok although the BER is
> probably higher then normal. My question is what to do with the bit
> stream now. What does the bit stream use for synchronization? Does it
> use start and stop bits? Is the stream encoded as ASCII or is it
> straight binary. The coast Guard WWW site says that it conforms to
> RTCM SC104 but I can not find any other info on the net. Does anyone
> have a copy of RTCM SC104 or the Coast Guard Broadcast Standards paper
> that can clear up these questions?
If you brave experimenters work somehting out, let us all know, please!
I've got an AEA PK232 as well, and my trusty Kenwood TS440. Very
unportable, but it would still be fun to try. I guess it could all go
in a car and run off the car battery.
--
Jud Ahern jah...@uoknor.edu Amateur Radio: KC5RI
Geology & Geophysics, U. of Oklahoma, Norman, OK 73019
"Opinions expressed here reflect the entire
University, in one convenient location."
Mr. Brooke Clarke
Judson L. Ahern wrote:
>
> Barbara Fehner wrote:
> >......... Does anyone
> > have a copy of RTCM SC104 or the Coast Guard Broadcast Standards paper
> > that can clear up these questions?
Hi:
Just take the RS-232 signal and feed in into your DGPS ready receiver at the baud rate
the GPS receiver wants to see. This may be the main problem since the data from the beacon
is usually 100 or 200 baud and most receivers want only one baud rate such as 4800.
Judson, how far are you from Pt. Loma?
Have Fun,
Brooke
N6GCE
Barbara Fehner
Brooke, The problem is that you need the format of the serial bit
stream off the radio in order to convert it to parallel bytes. Once
you have the parellel bytes then you can convert the baud rate and
send it to the GPS via a RS-232 stream. I have tried various
combinations of start & stop bits, and word length but with no
success. What is needed is the info in RTCM SC104 however it is $50 or
so.
BTW, I am about 40 miles from Pt. Loma.
Barbara
E-Mail: feh...@cts.com
__________________________________________________________
Richard Cochran
A couple of weeks ago I wrote a post describing how I was planning on
home-brewing a DGPS beacon receiver by hooking up an inexpensive FSK
decoder (MFJ 1225, designed for HF RTTY) between my shortwave receiver
and Garmin GPS45. A couple of people told me it might work, and a
couple of people told me it wouldn't, but I got the hardware and tried
anyway. Unfortunately, the people who discouraged me were right, at
least so far. But I promised to post results, so here they are.
My shortwave is receiving the beacon with an extremely strong, clear
signal, since I'm about 5 miles from the US Coast Guard DGPS beacon at
Pt. Blunt, Angel Island, San Francisco Bay. I can almost see the
beacon site from my window. So that's not the problem.
The decoder is designed for various frequency shifts, the lowest of
which is 170Hz. However, there's a pot inside that can be tuned, and
by turning the pot, I can get it to appear to lock onto the MSK
signal. There's a "phase lock" LED that stays locked on, and a "data"
LED that flickers. So far, so good, but I have no way of knowing how
well the decoder is tracking the data.
The decoder box outputs at standard TTL levels. There's no UART or
other timing circuit on the box; it's just sending the straight signal
out, at the speed the beacon is giving it. I assume the Garmin is
designed to receive the data as-is, since you have to set the beacon
speed on the receiver. I've set it for 200BPS, which is what the CG
says the Pt. Blunt beacon sends at.
My wiring is ok, which I've verified by watching an LED flicker when I
stick it across the appropriate pins on the Garmin connector.
I've tried inverting the signal and leaving it standard, no luck.
I've fiddled with the tuning on the shortwave and the shift pot on the
decoder, no luck. No matter what I do, I get the dreaded "No RTCM
Input" message from my Garmin.
I can think of three likely areas that are causing the failure:
1) The decoder just can't track the MSK signal accurately.
2) The Garmin needs something other than TTL level signals.
3) The RTCM data needs to be buffered and sent to the Garmin
at some speed other than the 200BPS that the CG sends it at.
I know I can do something about 2), but I think it's the least likely
to be the real source of the problem. I'm reaching here, but I think
I can figure out a way to do something about 3), but only if I know
exactly what needs to be done (baud rates, start bits, stop bits,
etc.). I don't have access to the right lab equipment (scopes, signal
generators) to verify 1) or to do much of anything about it.
If anybody can offer any help or suggestions, I'd appreciate it,
otherwise, I'll retire the decoder to be used for listening on the HF
bands, and I'll find another DGPS solution or I'll do without.
--Rich
Steve Haehnichen
-=> On Sat, 15 Jun 1996 19:16:54 GMT, rcoc...@netcom.com (Richard Cochran) said:
> A couple of weeks ago I wrote a post describing how I was planning
> on home-brewing a DGPS beacon receiver by hooking up an inexpensive
> FSK decoder (MFJ 1225, designed for HF RTTY) between my shortwave
> receiver and Garmin GPS45. A couple of people told me it might
> work, and a couple of people told me it wouldn't, but I got the
> hardware and tried anyway. Unfortunately, the people who
> discouraged me were right, at least so far. But I promised to post
> results, so here they are.
Thanks for posting. I am considering a similar sort of project
myself, using one of Kantronics' cheapo TNC boxes and a shortwave
radio.
It sounds like you've tried the obvious stuff.
Your best bet is to try to find someone local to you to lend a "real"
DGPS receiver. Once you establish that it works, take a peek at the
data signal coming out of it, and compare it to your own demodulated
data.
I don't have a DGPS receiver, but I have access to the rest of the
gear that you might need, like scopes, logic analyzers, or
heaven-forbid DSPs. :-)
I doubt the DGPS receivers do much more than repackage the data, like
you said. It shouldn't be hard to pick apart the data and see.
Alternatively, someone who has a copy of RTCM SC104 might be able to
shed some light on what differences exist between the radio beacon
signal and the DGPS data stream.
I have to think *someone* out there has the real spec. Too bad it's
not freely distributable. I'll see if I can find it at a local
library.
-Steve
Steve Haehnichen Vigra, Inc. San Diego, CA
st...@vigra.com (619) 597-7080 x169 Fax: (619) 597-7094
Lee Fraser
rcoc...@netcom.com (Richard Cochran) wrote:
>A couple of weeks ago I wrote a post describing how I was planning on
>home-brewing a DGPS beacon receiver by hooking up an inexpensive FSK
>decoder (MFJ 1225, designed for HF RTTY) between my shortwave receiver
>and Garmin GPS45.
Richard -
Thanks for sharing your experience. I'm in the process of designing
a receiver. So far I don't have very much information and certainly
nothing that I think would be new to you. However, through a friend
I hope to be getting some information on the correction beacon that
the Cnadian Coast Guard runs here on Partridge Island. I've asked
for info on the modulation scheme and anything else he can put his
hands on.
If you had a buffer I would think you could run the data into your
serial port on your PC. The buffer is needed to do the speed
matching. I don't think the serial port will adjust to the 100 or
200 baud transmitted by these stations. A 40B datascope (or more
modern equivalent) would also help.
Anyway, when this info shows up I'll post it so you can get a look at
it.
Lee Fraser
Bill Pechey
I think the TTL levels are the most likely unless your demodulator can't
handle 200 bit/s.
Any chance of looking at the data on an oscilloscope to see whether the
transitions all occur at the right moments?
Don't give up.
Bill.
Mark Fraser
The modulation is MSK, as in minimum shift keying. This is NOT the same
as FSK, which is Frequency shift keying. An fsk detector will provide a
bit-transition with changes in audio frequency.
MSK encodes bit transitions into changes in Phase,
I think , as well. No, I don't know enough to tell you how do do it,
but if you find out, I'm interested in the solution!
/mark
Bill Pechey (bpe...@cix.compulink.co.uk) wrote:
: I think the TTL levels are the most likely unless your demodulator can't
Mr. Brooke Clarke
Richard Cochran wrote:
>
> A couple of weeks ago I wrote a post describing how I was planning on
> home-brewing a DGPS beacon receiver by hooking up an inexpensive FSK
> decoder (MFJ 1225, designed for HF RTTY) between my shortwave receiver
> I can think of three likely areas that are causing the failure:
> 1) The decoder just can't track the MSK signal accurately.
> 2) The Garmin needs something other than TTL level signals.
> 3) The RTCM data needs to be buffered and sent to the Garmin
Rich:
The Garmin GBR 21 beacon receiver outputs data at either 4800 or 9600 baud.
I think that the Power/Data connector is designed for sending/receiving
RS-232 level signals. It looks you were correct on (2) and (3).
Page 45 of the Garmin 45 manual shows the RTCM/NMEA baud rate to be 4800.
Have Fun,
Brooke
Lee Fraser
rcoc...@netcom.com (Richard Cochran) wrote:
>A couple of weeks ago I wrote a post describing how I was planning on
>home-brewing a DGPS beacon receiver by hooking up an inexpensive FSK
>decoder (MFJ 1225, designed for HF RTTY) between my shortwave receiver
>and Garmin GPS45. A couple of people told me it might work, and a
>couple of people told me it wouldn't, but I got the hardware and tried
>anyway. Unfortunately, the people who discouraged me were right, at
>least so far. But I promised to post results, so here they are.
>My shortwave is receiving the beacon with an extremely strong, clear
>signal, since I'm about 5 miles from the US Coast Guard DGPS beacon at
>Pt. Blunt, Angel Island, San Francisco Bay. I can almost see the
>beacon site from my window. So that's not the problem.
>The decoder is designed for various frequency shifts, the lowest of
>which is 170Hz. However, there's a pot inside that can be tuned, and
>by turning the pot, I can get it to appear to lock onto the MSK
>signal. There's a "phase lock" LED that stays locked on, and a "data"
>LED that flickers. So far, so good, but I have no way of knowing how
>well the decoder is tracking the data.
>The decoder box outputs at standard TTL levels. There's no UART or
>other timing circuit on the box; it's just sending the straight signal
>out, at the speed the beacon is giving it. I assume the Garmin is
>designed to receive the data as-is, since you have to set the beacon
>speed on the receiver. I've set it for 200BPS, which is what the CG
>says the Pt. Blunt beacon sends at.
>My wiring is ok, which I've verified by watching an LED flicker when I
>stick it across the appropriate pins on the Garmin connector.
>I've tried inverting the signal and leaving it standard, no luck.
>I've fiddled with the tuning on the shortwave and the shift pot on the
>decoder, no luck. No matter what I do, I get the dreaded "No RTCM
>Input" message from my Garmin.
>I can think of three likely areas that are causing the failure:
>1) The decoder just can't track the MSK signal accurately.
>2) The Garmin needs something other than TTL level signals.
>3) The RTCM data needs to be buffered and sent to the Garmin
> at some speed other than the 200BPS that the CG sends it at.
>I know I can do something about 2), but I think it's the least likely
>to be the real source of the problem. I'm reaching here, but I think
>I can figure out a way to do something about 3), but only if I know
>exactly what needs to be done (baud rates, start bits, stop bits,
>etc.). I don't have access to the right lab equipment (scopes, signal
>generators) to verify 1) or to do much of anything about it.
>If anybody can offer any help or suggestions, I'd appreciate it,
>otherwise, I'll retire the decoder to be used for listening on the HF
>bands, and I'll find another DGPS solution or I'll do without.
>--Rich
I've found some info on the Coast Guard site that may be of interest:
1. Their DGPS uses selected existing marine radiobeacons in the 285 to
325kHz band.
2. There are two carriers. One uses MSK on a CW carrier. The other
is offset by 1.02kHZ above and carries a keyed identifier.
3. The frequenct shift is 25% of the bit rate so a 100 bps beacon
shifts +/- 25 Hz.
I have further info coming (I hope) from a contact in the Canadian
Coast Guard.
4. I don't know if the protocol can be directly fed to a pc or not.
I know it conforms to RTCM 104 but I don't have a description yet.
(If anyone has a description or knows something about it I would
appreciate a post).
That's all I have for now.
Lee Fraser
Michel Stoop
Mark Fraser wrote:
>
> The modulation is MSK, as in minimum shift keying. This is NOT the same
> as FSK, which is Frequency shift keying. An fsk detector will provide a
> bit-transition with changes in audio frequency.
I think you are refering to AFSK (Audio Frequency Shift Keying)
FSK changes the carrier frequency.
> MSK encodes bit transitions into changes in Phase,
MSK is a FSK signal with a minium shift of 1/4 baudrate.
Modulation methodes wich use Phase changes are:
PSK, Phase Shift Keying
BPSK, Bi Phase Shift Keying
and QPSK, Quadrarure Phase shift keying. Also known as vector modulation.
Michel
Michel Stoop
Mark Fraser asked me this:
How to demodulate the beacon MSK.
Wel there are 2 ways to do that.
You can use a SSB receiver and use an AFSK demodulator to recover the bitstream.
or use a FSK detector connected to the IF signal from a communications receiver.
For this you can use a Phase Detector or a Costas-loop PLL
I know nothing of the data format, and what must go to the garmin unit.
Presumably they transmit Satellite number and x,y,z offset values.
Sinse they are using 200bps I think that the datastream is synchronous
and packed in frames. And that the data is send in async (rs232) frames to the
garmin unit.
Since the datarate is 200bd the shift is 50Hz, very narrow!
This is why the garmin can retune the beacon receiver. Maybe there is also data
transmitted for tuning help. (ie transmitter drift value)
But I rember seeing 75Hz somewhere. But then the datarate would be 300bd.
Michel
Tom Clark (W3IWI)
The ~300 kHz beacons transmit MSK encoded binary data. The basic data formatting
is similar to that used with the GPS 50 b/s downlink, with 24 bit "words" and
6 bits of error checking, with the data highly compressed. See the GPS ICD-200
for the basic formatting spec, and then read the RTCM SC-104 document for details.
The data bit rates used at VLF are 50, 100 and 200 b/s, i.e. 1,2 & 4 times the
GPS satellite downlink rates. At 100 b/s, "fresh" DGPS info arrives about
every 8 seconds. The update rate for the commercial services using FM broadcasts
(i.e. DCS and AcquPoint) is faster than is sent by the Coast Guard.
The typical DGPS beacon receiver (this is also true of the FM broadcast RDS units)
convert this ugly binary data into "legal" ascii characters, sending 6 data bits
per 8-bit character, mapped into the <space>...A....Z.....a.....z character
space which is what your typical DGPS receiver wants to see (usually at 4880 b/s
rate). [When I have my W3IWI-13 packet DGPS becons on the air sending the bits
so that they can be copied with VHF amateur equipment, the DGPS data looks like
pure heiroglyphics, understood only by a DGPS-equipped GPS receiver. But the
characters are "user friendly" in that they don't cause the CRT of a causual
user sharing the frequency to flash, or send "beeps".]
The RTCM spec defines a number of DGPS message types sent at different rates. The
one with most of the info has the pseudorange and pseudorange-rate error for each
PRN in view at the differential station. Also sent are an ID number for the station
(0000-1023 in binary), the specific "serial number" (called IODE=Issue of Data
Ephermeris) sent by the GPS satellite to identify the ephemeris version in use,
and the typical GPS receiver will reject any info for which te IODE's don't match
(an IODE usually is applicable for ~6 hours time, depending on when the USAF
controllers uplink new data. So it is NOT unusual to have IODE mismatch).
All in all, the detailed format is pretty complicated and the "Cap'n Midnite
Magic Decoder Ring" code to generate or disentangle the DGPS messages runs
several pages of bit-diddling in "C". If there is REALLY a need for some code
to generate of decode the bits, and if someone are successfully decoding the MSK
bits from the beacons, I can probably put you in touch with a person (not me)
who has developed suitable software.
IMHO, kludging a DGPS beacon receiver that works will be a ~6 month intensive
effort -- not just something you can do with a TNC!
73, Tom