GPS NMEA output
Rick Filipkiewicz
description of the NMEA signalling levels and protocols used by GPS
units for output ? A web site would be ideal.
Thanks in advance
_________________________________________________________________________
Dr. Richard Filipkiewicz phone: +44 171 700 3301
Algorithmics Ltd. fax: +44 171 700 3400
3 Drayton Park email: ri...@algor.co.uk
London N5 1NU
England
Ian Strachan
<ri...@news.algor.co.uk> writes
>Anybody out there in net gliding land know where I can find a
>description of the NMEA signalling levels and protocols used by GPS
>units for output ?
Try Dickie Feakes at RD Aviation on rdavi...@delphi.com
--
Ian Strachan
i...@ukiws.demon.co.uk Bentworth Hall West
Tel: +44 1420 564 195 Bentworth, Alton
Fax: +44 1420 563 140 Hampshire GU34 5LA, ENGLAND
Martin Hoehle
: Anybody out there in net gliding land know where I can find a
: description of the NMEA signalling levels and protocols used by GPS
Try http://triumf.sundae.ca/pub/peter/ (mail me if i'm wrong with the URL,
i'm not VERY sure of it)
There should be a description of the NMEA protocol and data format, how to
hook up the GPS to your PC (for example) and some software that will display
or process the received data. (Basically NMEA data is transmitted 4800 baud,
8 bit, no parity, 1 stop bit serial).
Regards, Martin
_ _______________________________________________________ _
/ ) | | ( \
/ / | MARTIN HOEHLE University of Karlsruhe/Germany | \ \
_( (_ | _ irc: Flyby phone: +49 7222 52888 _ | _) )_
(((\ \> |/ ) ub...@rz.uni-karlsruhe.de ( \| </ /)))
(\\\\ \_/ / Martin...@stud.uni-karlsruhe.de \ \_/ ////)
\ / WWW: http://www.uni-karlsruhe.de/~Martin.Hoehle \ /
\ _/ \_ /
/ / |________ " Powered airplanes are for kids. " __________| \ \
/ / \ \
Rick Filipkiewicz
<snip ...
The (NMEA) physical setup is RS-232C at 4800 bps.
... endsnip>
I've seen GPS units and logger which define the NMEA outputs as +/-
which would imply differential RS422 instead of the single ended
RS232 ?
Frank van der Hulst
>Frank van der Hulst (fra...@pec.co.nz) wrote:
><snip ...
>The (NMEA) physical setup is RS-232C at 4800 bps.
>
>I've seen GPS units and logger which define the NMEA outputs as +/-
>which would imply differential RS422 instead of the single ended
>RS232 ?
I'm not sure what you mean by differential and single-ended in this
context, and I don't much know about RS422 (and I'm too lazy to find out
right now). But a 1 bit in RS232C is represented by a signal in the
range -3V to -15V, and a 0 bit is represented by a level between +3V and
+15V.
Anyway, I know that the NMEA output of a GPS _can_ be successfully
connected to a PC's RS-232 port, because I have some data collected
exactly this way.
--
Frank van der Hulst |You know you've landed gear-up
Embedded Software Engineer |when it takes full power to taxi.
PEC(NZ) Ltd |
Marton, New Zealand. |
Frank Smith
...................................
In <J8fDQCAv$o9w...@ukiws.demon.co.uk> Ian Strachan
<I...@ukiws.demon.co.uk> writes:
>
>In article <4cuq23$j...@moorgate.algor.co.uk>, Rick Filipkiewicz
><ri...@news.algor.co.uk> writes
>>description of the NMEA signalling levels and protocols used by GPS
>>units for output ?
>
I Johnston
: ><ri...@news.algor.co.uk> writes
: >>Anybody out there in net gliding land know where I can find a
: >>description of the NMEA signalling levels and protocols used by GPS
: >>units for output ?
Here's an article from the yacht-l mailing list a couple of years ago.
It's the best discussion of NMEA that I've found.
From: MAIL1::"SER...@acc.rwu.edu" 10-MAR-1994 21:19:52.61
To: I.JOH...@open.ac.uk
CC:
Subj:
For publication in the May 1991 issue of MAINSHEET magazine, the
magazine for owners of Catalina Yachts products: Catalina,
Morgan, Nacra, Prindle Capri, et. al. Information: CIS#
71311,2662 The author is the Catalina 27 Technical Editor
by Wayne Simpson
UNDERSTANDING MARINE ELECTRONICS INTERFACING:
the Promise, the Problems
The prospect of interfacing marine electronics has brought
with it a promise and a curse. The promise is that two or more
devices can share information and so become more valuable and
convenient to use. The curse is that not all marine hardware is
compatible. To make matters worse, marine electronics
manufacturers often make it difficult for the consumer to tell
whether any two devices will work together until bought and
wired together. Sometimes the result is a happy event. At other
times, it can be an exercise in frustration. An interface
is the boundary at which two independent systems communicate
and interact. When you connect an interface cable from your
loran to your autopilot, the wire is like the string between
two cans in a play telephone. The wire carries the conversation,
but it is the interfaces at each end that do the talking. If
the two devices speak the same language and can understand
the same words, then they can communicate. Otherwise, they
can't. There is a standard for communications in the marine
electronics industry, and most manufacturers claim to abide by
it, at least in part. Unfortunately this is not always enough.
NMEA 0183: The Marine Interface Standard
In 1980, a group of professionals from the industry met to
develop a standard "language" for marine interfaces. The result
was the National Marine Electronics Association (NMEA) 0180
standard. It addressed one problem, that of making lorans and
autopilots work together, and it was a success. In the following
years this standard, revised, broadened, then totally revamped to
take into account the wide range of electronics appearing on
boats, led to the current NMEA 0183 standard used on almost all
equipment today. The NMEA 0183 standard calls for data
communication in the form of coded "sentences." Each sentence
begins with the character "$" and ends with a carriage return
and line feed (<CR><LF>). These last two characters are
"control" characters and are not normally printed (for this
reason they are customarily shown enclosed in brackets).
Between the beginning and end of each sentence are "fields"
of data, a comma separating each field. The first field in
any sentence (field 0) begins with the two letter talker
mnemonic code ("talkers" are devices that send out information,
"listeners" take it in) followed by the three letter code
for the sentence. Data follows in the standard format for that
sentence. Here's an example:
field #: 0 1 2 3 4
sentence: $LCGLL,4001.74,N,07409.43,W<CR><LF>
What is being said here? Let's look at it field by field.
The sentence begins with the start character "$." Next comes the
talker identifier and sentence format code. LC stands for Loran-
C, GLL for present position in Lat/Lon. Field 1 contains the
set's current latitude. Field 2 is either N or S for North or
South. Field 3 is the set's current longitude. Field 4 is W or E
for West or East. Thus, this sentence reads "Loran-C present
position in Lat/Lon: 40 degrees 1.74 minutes North, 74 degrees
9.43 minutes West." A carriage return and line feed close the
sentence. The GLL sentence is always displayed in this format.
Each type of sentence, and there are many, has its own specific
standard format (for a brief catalog of formats, see sidebar:
Deciphering the Code). Standard sentences, each in a
standard format. This is how NMEA 0183 is intended to work. But
compliance with the standard is voluntary. It is up to the
manufacturers to decide how they will implement the standard,
and to what extent they will comply with it. Because the
standard is so broad, there is often more than one way to
express the same information, and this can cause problems.
There are many different sentences, and few if any talker
devices say them all, or adhere strictly to the standard format
of those they do. Does your autopilot need information in a form
that your loran isn't sending out? How can you tell? Don't
count on there being an adequate explanation in the manuals,
because often it isn't there. The owners manual for my loran
describes the workings of the interface this way. "The extra
connector on the rear panel is a serial data output interface
which can provide loran data to an autopilot, track plotter, or
fishfinder. The serial data is in NMEA 0183 format and is
transmitted continuously." That's it; the manual makes no other
mention of the interface anywhere. Other manuals I have seen are
about as descriptive. In a follow-up bulletin nearly a year
after I bought it, the manufacturer of my loran mentioned what
sentences the data output sent (there were four) and what the pin
assignments for the output connector were. This section was
labeled "for techies only." These interfaces are touted in
advertising copy, but it's clear the manufacturers don't really
expect you to use them. In the end, you probably will have to
call the manufacturer for a proper explanation of their
interface, and ask specifically whether they have tested the
devices you have in mind for compatibility. So far we've
discussed only software incompatibilities, but there can be
physical ones, too. NMEA 0183 specifies no standard connector,
and no two manufacturers seem to use the same one. Usually,
the owner will have to buy plugs from the manufacturers and make
his own cable.
A Case History
Two years ago, when I bought my West Marine Vector I loran,
I had no intentions of ever using the interface. Why would you
want to interface your electronics, anyway? Here's one example. A
loran can tell you where you are, what direction to steer to get
to your destination, and how far off your intended course track
you are. By itself, however, it can't steer the boat. An
autopilot can steer, but on it's own can't compensate for
currents or leeway. An autopilot interfaced with a loran can
do all these things, and then some. Here's another example.
The more you use the magic box, the more you realize the
information it gives should be displayed in the cockpit, where
the helmsman can have instant access to it. But the cockpit can
be a harsh location for electronics and most lorans aren't
truly waterproof. A loran "repeater," which echoes steering
information on a remote display, can provide useful steering
information in the cockpit while leaving the loran below in
the nav station, safe from sun and spray. An integrated
pilot/navigator system would have been nice, but LEGACY, our
1977 Catalina 27, already had an autopilot, an old style
Autohelm 800. It worked perfectly well and I wasn't going to
replace it just to have one with an interface. A loran repeater
would be handy too, but those available commercially cost as
much as a second loran. I toyed with the idea of building a
repeater but in the end dismissed the whole thing as not being
worth the time, effort, or cost. Two things happened last
summer that changed my opinion on interfacing. First, the old
Autohelm failed, and the cost to fix it was almost as much as a
new digital autopilot. I went for the new pilot, settling on a
Navico Tillerpilot 5000 largely because Autohelm had changed
their mounting dimensions and would have required me to change
my setup. Navico's dimensions were always very close the old
Autohelm's, and would fit without modifications. The new
pilot worked well all by itself, but an item on Navico's
option list caught my eye. It was a digital hand programmer
that made the TP5000 a much better pilot. It displayed present
heading from the pilot's internal fluxgate compass, allowed
you to make course changes in degrees rather than beeps, had an
off-course alarm and let you program the pilot's response
parameters to better tune them to the boat and conditions. Also
at the time, if you bought a pilot and programmer and sent the
receipts to Navico, they would send you the loran interface box
(which normally costs $150 at discount) for free. Second,
while at a marine warehouse sale, I came across a KVH model
LRX+ loran repeater. It was complete with the warranty card and
instruction booklet. It's price was $100, 1/3 the usual discount
price. It was NMEA 0183 compatible, and so was my loran. It even
had a sheet listing all the lorans the LRX+ had successfully
interfaced with. The Vector was there, right at the bottom of
the list. "Buy me," it crooned. Why not? I took it down to
my boat, hooked it up to the NMEA interface on the back of my
loran, programmed a course to a nearby buoy, and waited for
the repeater's display to come to life. It did, and showed a
bearing that was 12 degrees off! Something in the back of my
mind reminded me that the magnetic variation in my area was 12
degrees. The repeater was displaying bearing in degrees true
rather than magnetic. Every time I wanted to get a steering
course off the display, I would have to do a calculation in
my head. The "curse" had visited me.
I was now in the position of having to figure out what was
wrong with the repeater. I had no knowledge of the NMEA standard
or how it worked, so I called KVH. They said that whatever the
loran was sending out over the interface was reported on the
display. I called West Marine to find out what the Vector was
sending. They told me that both true and magnetic bearings were
sent, and that it was up to the repeater to differentiate between
them. I asked KVH for more help, and they offered to upgrade the
repeater's software to a newer version at no charge. I sent the
repeater off to KVH Industries in Middletown, RI. While the
repeater was away, I took a sample of the Vector's data output
using my PC. The procedure is described in the sidebar:
Listening In, Speaking Out. Here's what I got:
$LCGLL,4004.22,N,07409.78,W
$LCBWC,,4001.80,N,07403.66,W,117,T,129,M,005.2,N,011
$LCAPA,V,A,1.00,R,N,V,V,117,T,011
$LCVTG,,,271,M,02.3,N,,
The pattern of four sentences repeated itself every two
seconds. Close inspection of this string revealed the problem. In
order to work with a wide variety of lorans, the KVH repeater's
program accepts data from several different sentences, two of
which, BWC (bearing to waypoint along great circle) and APA
(autopilot format A), appear in the above sample. Look closely at
the APA sentence. Data fields 8 and 9 display the bearing and
whether it represents degrees true or magnetic. The standard
format for APA is degrees magnetic, in this example it is degrees
true. There was a bug in the Vector's software. Why
couldn't the repeater see that the bearing was in degrees true
and disregard it? Most of these programs are "comma counters."
They recognise a sentence of interest from the address, then
count commas until they get to the field in which they expect
to find data they need. They don't look at the fields telling
whether the information is true, magnetic, east, or west.
The people at KVH felt confident the software upgrade would cure
this condition, but when the repeater came back, my problem was
still there. I called West. The problem was their doing in the
first place, maybe they would fix it. Unfortunately for me, the
Vector I was made obsolete by the Vector II the previous
year. West planned no more software upgrades for the Vector I,
and it would be prohibitively expensive for them to rewrite the
software in response to this one complaint. I could, perhaps,
install the Vector II software (which corrected this bug) but I
would lose some of the features the Vector I had that the Vector
II lacked. I decided it wasn't worth it. There was still
some confusion over exactly what data fields the repeater was
looking at. To find out, I wrote a loran simulator program for
my computer (see sidebar). I coded the data in each of the
three fields that displayed bearing, giving each a different
value, and was able to prove that the repeater was looking at
the APA sentence for it's bearing to display. Once again, I
called KVH. Rob Solomon, one of KVH's technical people who had
stayed with me throughout this ordeal, was certain the
new software didn't look at APA for bearing. He wanted to
look further into the problem, but was unsure when he would be
able to find the time to do so. It was early February, war had
just broken out in the Persian Gulf and KVH was in the midst
of delivering 15,000 of it's DataScopes to Desert Storm
personnel. He promised to look at the LRX+'s software when things
quieted down. Oddly enough, he called back in an hour with the
LRX+'s program listing in hand. He told me absolutely that the
new software did not look at APA for bearing. Back to the drawing
board... I went home that night and ran my simulator again.
There was no mistake. I wrote a letter to Rob at KVH, including
everything I knew about the problem, a sample of the Vector's
data output, the listing of my simulator program, and the
version number of the software EPROM ("Erasable Programmable
Read Only Memory," the chip which stores the software) installed
in my repeater. I sent it off by FAX the following day. A few
days later, I called Rob to see if he had gotten the letter. He
had, but had not yet read it. When he did, he saw that I hadn't
been given the software he asked for. He sent a new EPROM chip
in the mail (the right one, this time) for me to put in
myself. I did, and it worked as promised. My problem was
solved, but only after five months had passed and I had spent
many hours on testing and investigation. I had better luck
regarding the autopilot. Late last summer, wisened by my
experience with the repeater (I had just begun the process of
sorting out it's troubles), I called West Marine and spoke to
Dave Wells, their technical support person for the Vector. I
was determined to find out if the loran/autopilot interface
would work before I bought it. He told me the Vector and the
Navico pilot were compatible. He had tested them himself. I
bought the programmer and sent away for the junction box. When
it arrived, the manual informed me that my pilot would require a
software upgrade from the factory in order to work properly with
my loran (sound familiar?). It was by then late fall and I would
not be needing the pilot for months, so I sent it off to Navico's
Largo, FL plant. When it came back, I connected the loran, pilot,
programmer and junction box together in my study to see what
would happen. The system worked as advertised. This time, the
"promise" was fulfilled.
Conclusions and Recommendations
My story has a happy ending. Come spring, I will be
installing my new toys and enjoying them all summer. Others who
try this may not be so fortunate. The NMEA system can work, but
there is no guarantee in any particular case that it will. My
repeater adventure shows what sort of problems you can have, and
to what lengths you may have to go in order to solve them.
My recommendations are these: Do your homework before you buy.
If you are buying from a supplier who will do the installation
for you, make sure he will guarantee the satisfactory operation
of the interface. If you will install the equipment yourself,
try to arrange for a full credit return in the event the
interfaces prove incompatible. Call the manufacturers and ask
questions. Take the time to understand what the NMEA 0183
standard is and how it works. If you find that you've been
stuck anyway all is not completely lost. A company called
Maricom Electronics, at 2911 River Drive, Thunderbolt, GA 31404
(phone (912) 354-4542) markets what it calls a "Universal
Marine Interface." For just under $400, this box claims to take
data in any format any talker might emit and put it into a
form any listener will understand. In the absence of less
extreme solutions, this may be worth trying.
SIDEBAR A:
NMEA 0183: DECIPHERING THE CODE
The "language" of NMEA 0183 is extensive but not difficult
to understand. The ground rules are these: maximum sentence
length is 80 characters including the starting "$," the
terminating <CR><LF>, and everything in between. Minimum number
of data fields is two, including the address field (talker
identifier and sentence format). If data for any field is
unavailable, a "null" field, two commas with nothing between
them (",,") can be sent instead. Here is a sampling of
two letter talker identifier codes. There are others, for
everything from satellite communications to atomic clocks, but
these are some of the most familiar to the recreational
sailor.
LC Loran-C
GP GPS
TR Transit SATNAV
AP Autopilot (magnetic)
HC Magnetic heading compass
RA Radar
SD Depth sounder
VW Mechanical speed log
Now for some of the more common sentences that might be
spoken by your LORAN, SATNAV, or GPS. This is not a complete
listing, but should illustrate how the system works. In the
following examples, the character "#" will denote some number
(0-9). Remember that each of these sentences would be preceded by
the start character "$" and the two letter talker ID. Each also
would be followed by the sentence terminator, <CR><LF>.
Geographic Location in Lat/Lon
field #: 0 1 2 3 4
sentence: GLL,####.##,N,#####.##,W
1, Lat (deg, min, hundredths); 2, North or South; 3, Lon; 4, West
or East.
Geographic Location in Time Differences
field #: 0 1 2 3 4 5
sentence: GTD,#####.#,#####.#,#####.#,#####.#,#####.#
1-5, TD's for secondaries 1 through 5, respectively.
Bearing to Dest wpt from Origin wpt
field #: 0 1 2 3 4 5 6
sentence: BOD,###,T,###,M,####,####
1-2, brg,True; 3-4, brg, Mag; 5, dest wpt; 6, org wpt.
Vector Track and Speed Over Ground (SOG)
field #: 0 1 2 3 4 5 6 7 8
sentence: VTG,###,T,###,M,##.#,N,##.#,K
1-2, brg, True; 3-4, brg, Mag; 5-6, speed, kNots; 7-8, speed,
Kilometers/hr.
Cross Track Error
field #: 0 1 2 3 4 5
sentence: XTE,A,A,#.##,L,N
1, blink/SNR (A=valid, V=invalid); 2, cycle lock (A/V); 3-5, dist
off, Left or Right, Nautical miles or Kilometers.
Autopilot (format A)
field #: 0 1 2 3 4 5 6 7 8 9 10
sentence: APA,A,A,#.##,L,N,A,A,###,M,####
1, blink/SNR (A/V); 2 cycle lock (A/V); 3-5, dist off, Left or
Right, Nautical miles or Kilometers; 6-7, arrival circle, arrival
perpendicular (A/V); 8-9, brg, Magnetic; 10, dest wpt.
Bearing to Waypoint, Great Circle
fld: 0 1 2 3 4 5 6 7 8 9 10 11 12
sen: BWC,HHMMSS,####.##,N,#####.##,W,###,T,###,M,###.#,N,#### 1,
Hours, Minutes, Seconds of universal time code; 2-3, Lat, N/S;
4-5, Lon, W/E; 6-7, brg, True; 8-9, brg, Mag; 10-12, range,
Nautical miles or Kilometers, dest wpt.
BWR: Bearing to Waypoint, Rhumbline, BPI: Bearing to Point of
Interest, all follow data field format of BWC.
For a full explanation of the NMEA 0183 standard, you can
write or call the NMEA. For a fee, they will send you the their
36 page booklet "NMEA 0183 Standard for Interfacing Marine
Electronic Navigational Devices." The address is: National
Marine Electronics Association, PO Box 50040, Mobile, AL 36605.
Phone (205) 473-1793.
Sidebar B:
LISTENING IN, SPEAKING OUT:
Using your Home Computer to Monitor NMEA Communications The data
that comes out of your loran or other NMEA equipped
gear is in the same format as that from your personal computer.
It is possible, and very easy to take a look at what your talkers
are saying, and to speak to your listeners in a form they can
understand. To listen in, you need a PC equipped with a
serial (RS-232) interface and modem program (such as HAYES
SmartCom or CTRM). If your program has a data capture feature,
you can read data from the line into a file for later use.
To make the connection, connect the NMEA signal line (line A,
output, etc) to the RS-232 "receive data" line (#2 on 9 pin or
#3 on 25 pin connectors), and the NMEA ground line (line B,
return, etc) to the RS-232 "signal ground" (#5 on 9 pin or #7 on
25 pin connectors). Transmission parameters for NMEA 0183
are as follows: 4800 baud, 8 data bits, 1 stop bit, no parity.
Set your modem program for these parameters. Switch on your
loran (or other talker), start the modem program, and the data
should come pouring in. One note of caution regarding
radionavigation receivers: they are very sensitive to the kind
of RF interference computers, fluorescent lights, TV's and
other such things create. Your receiver may not be able to
lock on to it's transmitters with these things operating
nearby, and you may have to set the receiver up in another room
and connect it with a long cable for it to work properly.
Speaking to your listeners is only slightly more complicated.
The data strings must be sent repeatedly every few seconds (but
no faster than once every second). For this, I wrote a short
simulator program in BASIC that does the job and allows me to
program in the text I want to send. The connection is different,
also. This time you will have to connect the NMEA signal line to
RS-232 "transmit data" (#3 on 9 pin or #2 on 25 pin connectors),
and NMEA ground to RS-232 signal ground (as before). In addition,
you probably will have to bring the RS-232 "data set ready" (DSR)
terminal to a low voltage. You can do this by inserting a jumper
between "data terminal ready" (DTR, normally #4 on 9 pin or #20
on 25 pin) and DSR (#6 on 9 or 25 pin setups). Here is the
text of my NMEA 0183 simulator program:
5 REM LORAN SIMULATOR PROGRAM - Wayne Simpson, 1991
10 OPEN "com2:4800,n,8,1,RS" FOR OUTPUT AS#1
15 REM set RS-232 parameters and disable request to send
100 A$="$LCGLL,4004.22,N,07409.78,W"
110 B$="$LCBWC,,4001.80,N,07403.66,W,100,T,200,M,005.2,N,011" 120
C$="$LCAPA,V,A,0.25,R,N,V,V,300,T,011"
130 D$="$LCVTG,,,268,M,02.1,N,,"
140 E$=""
200 PRINT A$ 'print GLL statement to screen
205 PRINT#1,A$ 'send GLL over interface
210 PRINT B$ 'BWC to screen
215 PRINT#1,B$ 'BWC to interface
220 PRINT C$ 'APA to screen
225 PRINT#1,C$ 'APA to interface
230 PRINT D$ 'VTG to scren
235 PRINT#1,D$ 'VTG to interface
240 PRINT E$ 'empty line to screen
245 PRINT#1,E$ 'empty line to interface
300 FOR I=1 TO 12000
310 NEXT
320 GOTO 200
330 REM line 300 provides the 2 second rep rate
340 REM line 320 repeats the transmission over and over
350 REM use ctrl-brk keystroke to terminate transmission
Load this program into your computer, substitute into the
A$-E$ variables whatever it is you want to say, and run it. You
are now speaking NMEA, and can do a fair job of troubleshooting
interface problems this way
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FROM: SER...@acc.rwu.edu
To: I.JOH...@open.ac.uk
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