MCFE Project Note:

Interfacing GPS Receivers and Hand-held Computers

Created: 7 May 1997
Last modified: 26 Jan 1998
This note covers the electrical interfaces between PCs, Newton MessagePad and Pilot computers and the following GPS receivers: The Garmin receivers output computed positions and are used in less demanding applications where low-precision location is acceptable. The Trimble and CMC devices both provide raw data outputs that can be used for differential post-processing. All three receivers can accept RTCM-104 differential correction input for real-time differential correction. Both Trimble and CMC receivers supply pseudorange output and the CMC ALLSTAR additionally provides carrier phase data, thus offering the possibility of sub-metre precision.
A version of the Trimble Lassen-SK8 is available with limited carrier-phase output, but this model has not been used in this project.

A useful comparison chart showing details of these and other receivers can be found on Sam Storm van Leeuwen's raw data pages, a very instructive resource for anyone interested in processing raw GPS data.

Garmin GPS-45XL, GPS-II and GPS-II+ hand-held receivers

The GPS45-XL and GPS-II are eight-channel hand-held receivers with a LCD display and buttons for control and user input.  The GPS-II+ is a similar device equipped with a more recently developed twelve channel receiver. The are suitable for stand-alone use as a simple navigational aid or can be connected to a computer via a serial interface.  These receivers normally output navigational data as NMEA 0183 v2.0 sentences and can accept input in RTCM-104 v2.0 format from a differential beacon receiver. In addition, they can also use a proprietary Garmin protocol that supports transfer of data such as track, waypoints and satellite almanacs.

They require a proprietary 4 pin socket to connect with their data lines and external power feed. Cables are available with this socket at one end and either a 9 pin D type socket, or bare wires at the other. The cable with female D type connector is suitable for direct connection to PC serial ports. The table below shows the required data connections for the PC 9 pin D type connector and the 8 pin mini-DIN connectors used for the Newton MessagePad serial port.

Garmin 4 pin PC 9 pin D Newton 8 pin 
mini-DIN
1 data in: PC to GPS (white) 3  3 
2 ground (black) 5  4 
3 data out: GPS to PC (brown) 2  5 
4 power 10-36VDC (red) n/c  n/c 
Note that the Garmin connector pins are numbered clockwise from the alignment key, looking at the fixed plug in the receiver, or anti-clockwise when looking at the free socket. The mini-DIN pins are numbered as follows when looking at the socket on the Newton. Note that there is an extended gap between the pins numbered here as 4 and 5:  
         8   7   6

       5      4    3

          2     1

Trimble SVeeSix-CM3 and Lassen-SK8 receiver cards

These receivers are designed for OEM use in embedded applications such as vehicle navigation systems. The SVeeSix is a well-established six channel receiver and the Lassen SK8 is a more recently introduced eight channel unit. Both are available in various versions differing in their output protocol and their ability to accept differential correction input on a second serial port. They are normally used with a miniature magnetic mount antenna (p/n 28367-00).

The receivers used in this project are:

Command input and data output for both of these is in Trimble Serial Interface Protocol (TSIP), a proprietary format employing binary command and report packets. Both accept RTCM-104 correction data input on the second serial port. On the SVeeSix-CM3, the second port is used for input only whereas the Lassen-SK8 outputs NMEA sentences on this port.

The Trimble receivers are supplied as small printed circuit cards with CMOS compatible TTL level input and output. For connection to the computer, the I/O lines must be inverted and buffered to RS232 levels. A simple interface circuit shown below provides a regulated power supply, backup power for the on-board memory and buffering for the main I/O lines. The 1 PPS timing signal is not used here, so has not been buffered.

The receiver card is mounted in a small plastic box (110x60x30mm) together with the interface and regulator circuit. The receiver antenna socket (SMB type) protrudes though one end of the box. The remaining connectors, two 9 pin male D type data connectors and a 2.1mm power socket, are mounted along one side of the box.

CMC Allstar receiver cards

The Canadian Marconi Company ALLSTAR is also designed for OEM use in embedded applications. It is a twelve channel receiver card and is available in several versions offering position, velocity and time outputs at rates from 1Hz to 10Hz.

Two serial ports are provided, one of which can be used for input and output of either NMEA format messages or the proprietary CMC binary protocol. The second port may be used for input of RTCM-104 differential correction messages. A range of proprietary command and report messages are available in NMEA format, but the binary protocol must be used for full access to pseudorange and carrier-phase data.

The interface circuit used in this project with the ALLSTAR is the same as that used with the Trimble cards. The receiver and interface is mounted in a plastic box (120x130x40mm) with a BNC antenna socket, two 9 pin male D type data connectors and a 2.1mm power socket mounted along one end.  Although CMC recommend the use of several other antennas, the device has only been used so far with the small Trimble antenna mentioned above.

Batteries and boxes

The boxes used to house the Trimble and CMC receivers fit easily into a pocket of a field jacket together with batteries and cables emerging for connection to the hand-held computer and hat-mounted antenna. The use of low-dropout regulators in the interface circuit allows us to use 7.2V rechargeable batteries. Various options have been used here giving different durations of use:

Interface Circuit

Components:

Connectors:  
Pin Numbers
Function Interface circuit Trimble 
CM3 and SK8		 CMC  Allstar (J3)
Port 2 TX
1
1
8
+5VDC
2
2
16 - Antenna (if required),
17 - Digital, 
19 - RF
(and 11 - VDD, if battery backup not used)
Port 1 TX
3
3
14
Backup battery +ve
4
4
11 - VDD (if battery used)
Port 1 RX
5
5
12
1 PPS timing pulse
6
6
5
Port 2 RX
7
7
7
Ground
8
8
9, 13, 18
  Connecting cables for either port:
Receiver 9 pin D 
(either port)		 PC 9 pin D Newton 8 pin 
mini-DIN
2  3  3 
3  2  5 
5  5  4 
 
Nick Ryan
Computing Laboratory, University of Kent at Canterbury, UK
N.S.Ryan@ukc.ac.uk