Z Technology, Inc.

GlobalPositioning
System Receivers

Application note

By: John Purdy

Z Technology, Inc.

December 18, 1997

Version 2

Table of Contents

Introduction:

What you need to know about GPS:

What you need to know about NMEA:

Serial Port connection

Garmin GPS45XL and Trimble SVeeSix Plus Comparison

GARMIN GPS45XL

Trimble SVeeSix Plus GPS

 

Introduction:

This application note describes the use of two NMEA compliant GPS systems, a Garmin GPS45XL, and a Trimble Navigation SVeeSix Plus. Both units have given favorable results with the Z Technology GPS compatible programs supplied on the Control Software diskette provided with each Z Technology Field Strength Meter. Included, is information about GPS, NMEA, and some web site URL's (addresses) to look at for more in depth explanation of each. A description of differences between the units two GPS units. A procedural description of connecting each of the GPS units to a laptop where a Z Technology Field Strength Meter, is also connected, and the laptop is running a GPS compatible Z Technology Control Software program. Finally each GPS units specifications are listed.

What you need to know about GPS:

What is GPS?

GPS stands for Global Positioning System. It is a navigational system put in place by the U.S. Department of Defense. It consists of 24 satellites that orbit the earth, each satellite broadcast signals containing time and identifier codes. Through an antenna, GPS receivers pickup and interpret the signals from multiple satellites to determine position, altitude, speed, and direction of travel of the GPS unit. In order for a GPS receiver to locate itself, it must have clear reception of at least 3 satellites. By measuring the distance from three satellites it can "triangulate" it's position anywhere on earth. With a clear view in a flat area a GPS unit may have reception of up to eight satellites. The satellites are arranged in space such that at any time any place on earth at least 5 satellites should be visible. Reception of four satellites allows accurate altitude determination and correlation to universal time. Mountains, buildings, and even trees can block low power Giga Hertz GPS signals.

GPS accuracy is controlled by the US Department of Defense. It is considered a matter of national security, although there are some discussions occurring which may change that by the year 2005. Some error is introduced by a government controlled function called Selective Availability, or SA. Civilian GPS units are all subject to SA. At Z Technology we use civilian GPS units. With the Selective Availability active, Standard GPS is guaranteed to be within 100 Meters 95% of the time. The Department of Defense controls the amount of error introduced by Selective Availability. In actual practice Standard GPS with SA active is generally accurate to about 50 meters, (~150 feet.)

What is Differential GPS, DGPS ?

Above we have described Standard GPS. Differential GPS is a method by which civilians can get more accurate location information. It effectively removes errors introduced by the GPS radio signals propagating through the ionosphere, and the government introduced Selective Availability. A fixed GPS receiver, at a known location is set up to calculate the error in the GPS signal coming from the satellites. It then broadcasts a correction over a low frequency carrier, typically around 300kHz. Mobile differential ready GPS units can be connected to a differential correction receiver via a serial port. The differential ready GPS unit will then output differentially corrected positions. With DGPS, positional accuracy improves to about 5 meters. Both the Garmin and Trimble units discussed here are differential ready.

In US coastal regions the US government provides differential correction signal for public use. In other areas there are normally private companies that provide a differential correction signal, and rent differential correction receivers. Companies such as Trimble Navigation also sell differential correction transmitters, and receivers.

There are surveying systems available today, which use carrier phase correction GPS technology that are accurate to 2 cm. These units are beyond the scope of this discussion. They are very specialized, and very expensive. The bottom line is the more accurately you want to know your position, the more it will cost.

A good source of public information about GPS, GPS Receivers, and Map Programs is Peter Bennett's FTP site ftp://sundae.triumf.ca/pub/peter/index.html. Down load the FAQ's.

There is also a very good easy to understand tutorial on GPS and GPS technology at http://www.trimble.com/gps/.

What you need to know about NMEA:

What is NMEA?

NMEA stands for National Marine Electronics Association. It is a supervisory board that has published specifications describing how many different types of navigational equipment devices can communicate with one another over a serial RS-232 type connection. There are a number of generations of the specification available. Today we use NMEA 0183 version 2.0.

A GPS unit can use a number of different communication protocols to communicate with other devices. Some protocols are proprietary, and very special purpose. Others are public, more general purpose, and therefore supported by numerous devices. The NMEA 0183 version 2.0 is a protocol which has been accepted by many different manufacturers. The GPS compatible programs supplied with your Z Technology Field Strength Meter use information from a GPS in the NMEA 0183 version 2.0 standard. NMEA compliant devices broadcast groups of information called sentences via an RS-232 serial port.

What does a NMEA sentence look like?

Each NMEA sentence starts with a $ followed by five characters that define the source device, and what kind of sentence it is. Different sentences are intended for different purposes, and contain different information fields. Each field is separated by a comma, for ease of parsing. The Z Technology GPS compatible software uses a $GPGGA, $GPGLL, or $GPRMC sentence to obtain Latitude and Longitude information. Since Z Technology provides source code, you can modify the programs to parse any sentence for Latitude and Longitude information. The Control Software programs defaults to using $GPGGA sentences, both the Trimble and Garmin units described in this appnote also include the $GPGGA sentence in their default group of broadcast sentences.

Below is a typical GPS NMEA 0183 version 2.0 broadcast Group. These came from a Garmin GPS45XL. The Garmin GPS45XL produces these broadcasts every two seconds.

$GPRMC,211941,A,4531.931,N,12251.211,W,000.0,059.0,080497,018.7,E*65

$GPRMB,A,,,,,,,,,,,,V*71

$GPGGA,211941,4531.931,N,12251.211,W,1,04,2.5,136.1,M,-20.4,M,,*7C

$GPGSA,A,3,,04,05,,09,,24,,,,,,5.1,2.5,4.4*3F

$GPGSV,2,1,08,02,13,111,00,04,56,126,40,05,39,298,46,07,40,058,00*75

$GPGSV,2,2,08,09,59,254,42,16,07,074,00,24,34,173,46,30,08,294,00*70

$PGRME,58.5,M,104.1,M,119.4,M*1F

$GPGLL,4531.931,N,12251.212,W,211942,A*39

$PGRMZ,446,f,3*1D

$PGRMM,WGS 84*06

$GPBOD,,T,,M,,*47

$GPRTE,1,1,c,0*07

Next is a typical Trimble SVeeSix Plus broadcast.

$GPGGA,005406,4531.920,N,12251.166,W,1,3,002.3,,M,-020,M,,*46

$GPVTG,000,T,341,M,00.0,N,00.0,K*48

Note that the Trimble SVeeSix Plus outputs only $GPGGA and $GPVTG sentences. The transmission rate dictated by the NMEA standard is 4800 baud. 1 stop bit, No Parity. At this baud rate, the multitude of sentences put out by the Garmin unit requires more than one second of transmission time. The Garmin unit's update rate is once every two seconds, the Trimble unit's update rate is once per second. Using a proprietary Trimble protocol, TSIP (Trimble Standard Interface Protocol,) the update rate is twice per second. TSIP is not currently supported by the Z Technology Control Software programs.

For more information on NMEA contact:
National Marine Electronics Association
P.O. Box 3435
New Bern NC, 28564-3435
Ph: 919-638-2626 FAX: 919-638-4885
http://www.nmea.org

Serial Port connection

Using the Z Technology Field Strength Meter with a laptop computer, and a GPS system will require two serial COM ports on the Laptop. One to connect to the Z Technology Field Strength Meter, a second to connect to the GPS. The system should be connected as illustrated below.

Z Technology: Serial Port Connections

FIGURE 1. Serial Port Connections

A typical laptop has a built in serial port, generally configured to be COM1 and a modem to be COM2. The modem configuration will have to be changed to use either COM3 or COM4. If your laptop has an infrared port configured to use COM1 or COM2, it will also need to be disabled, or re-configured to use COM3 or COM4. See your manufacturers documentation for instructions.

Few laptops come configured with two externally available serial ports. Likely, you will have to add a PCMCIA card to obtain a second serial port. The GPS software that comes with your Field Strength Meter is written in QBASIC. QBASIC is limited to using COM1 and COM2. Your laptop serial ports will have to configure to COM1, and COM2. Since QBASIC programs are DOS based programs, you will find that they run better and faster in a DOS environment, then a Windows environment. When configuring the serial ports, you must ensure that they will operate correctly in the DOS environment. For a PCMCIA serial card, this typically involves loading a driver via the config.sys and/or the autoexec.bat files.

Independent verification that the serial ports are working properly, and that the GPS is sending out sentences is possible by running a terminal emulator on your computer such as ProComm when running in DOS, or HyperTerminal when running in Windows 95, and connecting the GPS to the port in question. The terminal emulator will display the GPS sentences if everything is working correctly.

The NMEA standard dictates that the following RS-232 settings be used.

NMEA 0183 Ver. 2 RS-232 Settings

Baudrate

# of Stop Bits

Parity

4800

1

None

The Field Strength Meter can be connected to COM1 or COM2 of the Laptop. Z Technology programs default to the Field Strength Meter connected to COM1.

The GPS can be connected to COM1 or COM2 of the Laptop. Z Technology programs default to the GPS connected to COM2.

Socket I/O has been found to be a suitable supplier of PCMCIA serial port cards.

Garmin GPS45XL and Trimble SVeeSix Plus Comparison

The Garmin is the lower cost of the two units. It is a hand held unit 6" x 2"x 1.25", it easily fits in a shirt pocket. It is designed for personal navigation use on a water vessel. It is water proof, nitrogen filled. It is available from marine supply stores for less then $400.00 USD. It can be powered remotely or by 4 AA batteries. Both power and serial port connection is via a non standard DIN like round 4 prong connector. A cable, available separately, is simply a pigtail to the four prong connector. The user must properly apply connectors to the pigtail end. The Garmin unit comes with a detachable passive antenna. The antenna can be remotely mounted with a 2 meter coax cable with BNC connectors (available separately.) Attachment to the roof of a vehicle is via a suction cup. The Garmin has a 64 x 100 pixel LCD which displays a number of different screens some of which include Latitude, Longitude, speed, direction of travel, way points, maps, etc. It has a single satellite tracking receiver that is time multiplexed 8 ways to track the satellites. It contains velocity prediction software that continues to output GPS sentences for up to 20 seconds after it has lost the satellites. This is handy for going through short tunnels, but if it doesn't regain the satellites in that 20 seconds, it takes another 20 seconds to start out putting LAT and LON data. The velocity prediction software also tends to make the recorded path over shoot the actual path at turns. The short term stationary drift of the Garmin is about 10 times that of the Trimble.

The Trimble SVeeSix Plus, is designed for industrial use. It comes in a 5" x 4" x 0.6 " aluminum sheet metal dust proof enclosure with mounting holes. It has two standard mini DB9 connectors for communication. A third connector for the FOG active magnetic mount antenna, is powered from the SVeeSix Plus, it has a 5 meter cable. A fourth connector is for power and battery backup. The power connector is a round three prong connector with a twist lock. The unit comes with a pigtail which connects to the power twist lock connector and may be wired into the users 12v automotive system. The SVeeSix Plus is available from Trimble Navigation as an OEM unit (if you need to purchase quantities), Part #34093-62. OEM price varies. The antenna is a separate item also an OEM unit, Part #21423-00. Z Technology resells these units for $650.00 and $159.00 respectively. The SVeeSix Plus is the same electronics used by General Motors in their vehicle locating products. It is intended to be used entirely via the serial port, it has no display. It has no accommodation for housing batteries. It contains six satellite tracking receivers. It has very little short term stationary drift. IE when the vehicle is stopped for a short time measured in seconds, the LAT and LON remains constant. Long term drift by both units is determined by the SA, and has been observed to be approximately the same.

GARMIN GPS45XL

If you choose to use the Garmin GPS45XL receiver, you will need the following cables. They can be purchased direct from Garmin.

Garmin International
1200 E. 151st Street Phone: (800) 800-1020
Olathe, KS 66062 USA FAX: (913) 397-8282
Power Data Cable # 010-10082-00 $27.96 USD
Remote Antenna Mount Cable # 010-10144-00 $25.45 USD
(as of June 1997)

The Power Data Cable allows connection of the GPS receiver to the computer, and an external power source. The Remote Antenna Mount Cable allows the detachable Garmin GPS45XL antenna to be attached to the outside of a vehicle.

The Power Data Cable plugs into the round DIN like connector on the back of the Garmin GPS45XL. The other side of the cable is a pigtail. You will need to connect it to a female DB-9 or DB-25 connector as shown in the following table. You will also need to connect the power and ground appropriately if you wish to not use batteries.

GARMIN GPS45XL POWER DATA CABLE CONNECTIONS

Function Garmin Color DB-25 Socket

(female)

DB-9 Socket

(female)

Data In White 2 3
Data Out Brown 3 2
Ground Black 7 5
Power (10-40VDC) Red -- --

Z Technology: Garmin GPS45XL Rear Connector

FIGURE 2. GARMIN GPS45XL REAR CONNECTOR

If you are unable to order and modify the above Power Data Cable, Z Technology can supply you with a modified GARMIN GPS45XL Power Data Cable. The modified cable comes complete with a 9 pin connector to connect to your lap top, and a power connector that will allow you to use the same type of charger block you use for your Field Strength Meter to power the GPS. (Contact Z Technology for more information.)

When placing the antenna on the test vehicle, a center position on the roof will give optimum reception. The GPS will not receive the satellite signals in tunnels, under full foliage trees, or in some "urban canyons" where tall buildings shield the antenna from line of sight view of the satellites.

Once cables have been assembled and installed, verify that the GPS is out putting sentences, and that the laptop serial ports are working correctly. Use the procedure described in the Serial Port Connection section earlier in this appnote.

Connect the GPS, and Field Strength Meter to the laptop as shown in Figure 1. Refer to the Application Note: GPS Compatible Control Software for instructions on using the Z Technology GPS compatible software.

Garmin GPS45XL Specifications:

Physical:
Case: Waterproof, dry nitrogen-filled
Size: 6.15H" x 2"W x 1.23"D

(15.6 x 5.1x 1.23 cm)

Weight: Approx. 9.5 ounces

(269g) with batteries

   
Temp Range: 5o to 158oF ( -15o to 70oC)
Performance:
Receiver: Differential Ready MultiTrac8TM
Acquisition Time: ~20 seconds (warm start)

~ 2 minutes (Cold Start)

~ 7.5 minutes AutoLocateTM

Update Rate: 1/second continuous
Position Accuracy: 5-10 meters (16-33 ft.) with DGPS correction*

15 meters (49 ft.) RMS**

Velocity Accuracy: 0.1 knot RMS steady state(90knots/103mph max.)
Dynamics: Performs to specifications to 3g's

* With optional Garmin GBR21 Beacon Receiver Input.

** Subject to accuracy degradation to 100m 2DRMS under the US DOD-imposed Selective Availability Program.

Power:
Input: Four 1.5 volt AA batteries or 10-40 volt DC
Current Consumption: 0.16 amps max.
Battery Life: Up to 12 hours (normal mode)

Up to 20 hours (w/ alk. batteries, battery saver mode)

Trimble SVeeSix Plus GPS

Z Technology re-sells the Trimble SVeeSix Plus and FOG active magnetic mount antenna with all necessary cabling. Connect the power pigtail to a +9 to +32 V DC source, connect the antenna, and the unit is operational. The power connector is a three prong power connector for DC power, backup power, and ground. The pinout is shown in the table and diagram below.

Pin #

Color

Signal

Power Draw

1

Yellow

Battery backup

<100uA 2.6uA @ 3.5 V

2

Red

Power +9 to +32V

1.5 to 2.0 Watts

3

Black

Ground

 

Z Technology: Trimble SVeeSix Plus Power Connector

FIGURE 3. Trimble SVeeSix Plus Power Cable Connector

From the time the SVeeSix Plus receives power it will begin to transmit NMEA sentences. It normally takes two to five minutes to get a solid fix from power up. This time can be reduced by connecting a battery to the Battery Backup Power lead. This allows the GPS to remember important almanac information. Z Technology GPS compatible programs display a "POOR COVERAGE" message when the GPS is transmitting a message indicating it has not yet acquired a proper fix or has lost view of the satellites. Wait for the unit to locate itself, or take action to insure the GPS antenna has a clear view of the sky.

There are two data ports on the Trimble SVeeSix Plus Port 1, and Port 2.

When using NMEA communication protocol, Port 1 is used.

For Standard GPS, a standard "straight through" cable with a DB 9 pin male connector on one side and a DB 9 pin female connector on the other side can be used to connect the GPS to the Laptop.

If Differential GPS (DGPS) is required a special cable is needed to interface Port 1 of the GPS, to both the differential correction receiver, and the laptop. The GPS will transmit serial data on Pin 2 of Port 1, it will receive differential correction data on Pin 3 of Port 1.

Cable for Differential Correction GPS with NMEA Protocol

To GPS Port 1
DB9 Female

To Laptop
DB9 Male

To Differential Receiver
DB9 Male
1 NA 1  
2 XMIT 2  
3 RCV   3
4 NA 4  
5 GND 5 GND 5 GND
6 NA 6  
7 NA 7  
8 NA 8  
9 1 Pulse/Sec. 9  

The Trimble SVeeSix Plus is used with a FOG active magnetic mount or bulk head mount antenna. The name FOG, has nothing to do with cold damp weather, it was merely an internal code name during development. Its 5 meter cable carries power from the GPS receiver to an amplifier in the antenna and carries the amplified signal from the antenna to the receiver.

When placing the antenna on the test vehicle, a center position on the roof will give optimum reception. The GPS will not receive the satellite signals in tunnels, under full foliage trees, or in some "urban canyons" where tall buildings shield the antenna from line of sight view of the satellites.

Verify that the GPS is out putting sentences, and that the laptop serial ports are working correctly by using the procedure described in the Serial Port Connection section earlier in this appnote.

Connect the GPS, and Field Strength Meter to the laptop as shown in Figure 1. Refer to the Application Note: GPS Compatible Control Software for instructions on using the Z Technology GPS compatible software.

Trimble SVeeSix Plus Specifications:

GPS Receiver

General

L1 frequency (1575.42 MHz, C/A code (Standard Positioning Service), 6-channel, continuous tracking, differential ready

Positional Accuracy

DGPS = 2 to 5 meters (2 Sigma) steady state conditions with differential

Standard = 15 meters (SEP) steady state conditions without selective availability

Velocity Accuracy

0.1 m/sec. (1Sigma) without selective availability

Time Accuracy

1 microsecond with 1 pulse-per-second output synchronized to UTC

Datum

WGS-84 (220 options and one custom slot)

Acquisition Rate

Momentary signal interruption: 2 seconds typical

Momentary power interruption: less than 30 seconds with RAM battery backup

Cold Start: 2- 5 minutes

Dynamics

Velocity: 500 m/sec/ (max)

Acceleration: 4g (39.2m/sec2)

Jerk: 20 m/sec.3

Environmental Characteristics

Temperature

Receiver:

Standard Operating, -10oC to +60oC

Storage, -55oC to +100oC

Antenna:

Operating, -40oC to +85oC

Vibration

0.008g2/Hz 5Hz-20 Hz

0.05g2/Hz 20Hz-100Hz

-3dB/octave 100Hz-900Hz

(Comply with SAE J1211 requirements)

Altitude

-400 to +18,000 meters MSL

Humidity

95% R.H. condensing @ +60oC

Physical Characteristics

Size and Weight

Receiver:

4.97" W x 4.03"D x 0.62"H

(127mm x 102mm x 28mm)

0.57 lbs. (260 g)

Antenna:

2.35"(60mm) diameter

0.76" 1.93mm height

0.13 lbs. (60g)

Power

Receiver and Antenna:

+9 to +32v DC, 270milliamps, 1.5 watts minimum.

Micro Conxall connector, 3 pin male socket plug

RAM Backup: +3.5 to +14 v DC via power cable

Input/Output

Interface

On-board serial port interface through 9-pin connector. Serial port operates at RS-232 or RS-422 levels

Protocols Available

TSIP: Trimble Standard Interface Protocol

TAIP: Trimble ASCII Interface Protocol, NMEA 0183

One Pulse Per Second

Timing

Leading edge of pulse synchronized to UTC within 1 microsecond, nominal

Pulse Width

1 microsecond wide pulse: falling edge is 20 nanoseconds or less, depending on distributed capacitance in cable

Output

Open-collector

RF Interference

Jamming

Resistant to broad band noise jamming where jamming-to-signal power ration is 20 dB or less, measured at the antenna/preamplifier interface when input signal is at -160 dBW

Burnout

Protected from damage by RF signals at frequencies 100MHz or more from the L1 frequency (1575.42) with received power up to one watt at the antenna