Brandywine GPS4 User manual

MANUAL P/N 900000038 REV 2.2
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User Guide
GPS4 Satellite Synchronized Time & Frequency
Standard
Model GPS4
P/N 001-0045
Revision 2.2
November 2004
Brandywine Communications
1153 Warner Avenue
Tustin, CA 92780
[phone]
[phone]
http://www.brandywinecomm.com

MANUAL P/N 900000038 REV 2.2
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Revision History
REVISION DATE COMMENTS
2.1 08-29-03 Preliminary release of GPS4 user guide.
2.2 11-02-04 Revision of entire GPS4 user guide.

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Table of Contents
1 Introduction............................................................................................................................... 5
2 Product Specifications.............................................................................................................. 6
2.1 Power Requirements......................................................................................................... 6
2.1.1 Standard .................................................................................................................... 6
2.1.2 Power Option 1 .......................................................................................................... 6
2.1.3 Power Option 2 .......................................................................................................... 6
2.2 Outputs.............................................................................................................................. 6
2.2.1 PPS ............................................................................................................................ 6
2.2.2 10 MHz....................................................................................................................... 7
2.2.3 Synthesizer ................................................................................................................ 7
2.2.4 Status Indicators ........................................................................................................ 8
2.3 Inputs................................................................................................................................. 8
2.3.1 System Reset............................................................................................................. 8
2.3.2 Antenna...................................................................................................................... 8
2.4 Environmental ................................................................................................................... 8
2.5 GPS Receiver ................................................................................................................... 8
2.6 Serial Control .................................................................................................................... 8
2.6.1 Control Port................................................................................................................ 9
2.6.2 TOD Port.................................................................................................................... 9
2.6.3 Serial Option 1 ...........................................................................................................9
2.6.4 Serial Option 2 ...........................................................................................................9
2.7 Hardware Status ............................................................................................................... 9
2.8 Physical ............................................................................................................................. 9
2.9 Connectors and Indicators .............................................................................................. 10
3 Installation .............................................................................................................................. 11
3.1 Thermal Considerations .................................................................................................. 11
3.2 Power Supply Connections ............................................................................................. 12
3.3 Unit Mounting .................................................................................................................. 13
3.4 Serial Interface Connections ........................................................................................... 14
3.5 Interfacing to the Serial Port............................................................................................ 14
3.5.1 RS232 ...................................................................................................................... 14
3.5.2 TxD2 ASCII Time of Day Output.............................................................................. 15
3.5.3 RS422 ...................................................................................................................... 15
3.6 Antenna Installation......................................................................................................... 15
3.6.1 Location ................................................................................................................... 15
3.6.2 Exposure to High RF Fields..................................................................................... 15
3.6.3 Lightning Protection ................................................................................................. 16
3.6.4 Antenna Cable Factors ............................................................................................ 16
3.6.5 RF Loss.................................................................................................................... 16
4 Operating Instructions ............................................................................................................ 17
4.1 Scope of Section ............................................................................................................. 17
4.2 Operation......................................................................................................................... 17
4.3 Serial Data Communication ............................................................................................ 20
4.4 Control TxD1/RxD1 ......................................................................................................... 20
4.4.1 TxD1 / RxD1 User Commands and GPS4 Responses ........................................... 21
4.5 Message Formats ........................................................................................................... 24
4.5.1 Time and Date with Status, Position Averager Status, and Output Status.............. 24
4.5.2 Pulse Output Data and Command........................................................................... 26
4.5.3 Dilution of Precision Values and Satellites Used ..................................................... 29
4.5.4 Time Code and Serial Data Output Formats ........................................................... 30
4.5.5 Frequency and Phase Controller Data .................................................................... 33
4.5.6 Health Status of Satellites........................................................................................ 39

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4.5.7 Leap Second Information......................................................................................... 40
4.5.8 Location and Signal-Noise Ratio of Satellites.......................................................... 41
4.5.9 Firmware Version Number....................................................................................... 42
4.5.10 Offset of Local Time Data and Command ............................................................... 43
4.5.11 Position of GPS Antenna ......................................................................................... 45
4.5.12 Software Reset Command....................................................................................... 46
4.5.13 Additional Information .............................................................................................. 47
4.5.14 Request Precision Time........................................................................................... 48
4.6 TxD2/RxD2 Messages .................................................................................................... 50
4.7 IRIG B and AFNOR NF 2 87-500 Time Code Output ..................................................... 52
4.8 NASA36 Time Code Output Option ................................................................................ 55
4.9 2137 Time Code Output Option ...................................................................................... 57
4.10 VELA Time Code Output Option ..................................................................................... 58
5 Maintenance and Calibration.................................................................................................. 59
5.1 Scope of Section ............................................................................................................. 59
5.2 Routine Maintenance ...................................................................................................... 59
5.3 Fault Finding ................................................................................................................... 59
6 Diagrams ................................................................................................................................ 60

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1 Introduction
The GPS4 disciplined time and frequency standard is a device that offers many
desirable features. Utilizing signals from the Global Positioning System, the unit
provides highly accurate time, synchronization, and frequency functions. The
unit may operate in either GPS or UTC time mode. Local time offsets may be
programmed into the unit in resolutions of minutes. A brief outline of the unit’s
features is described below.
•PPS Output: This 1 PPS output provides a TTL output level into 50 ohms.
This output is available on the front panel BNC connector labeled “PPS”.
•10 MHz Output: This 1 Vrms sinusoidal output is available on the front
panel BNC connector labeled “10MHz”.
•Synthesizer Output: This is a frequency output option that is factory
configurable to produce one of three different frequencies (19.6608 MHz,
1.544 MHz, and 2.048 MHz). This output is available on the front panel
BNC connector labeled “SYNTH”. Other frequencies may also be
synthesized so please contact Brandywine Communications for more
information on a particular frequency.
•IRIG B Time Code Output: This is a time code output used to distribute
time of day information to other devices. This output is available by an
internal link selection (DC level shift code output or modulated time code
output). It is available on the front panel BNC connector labeled “CODE”.
Other time code formats are possible so please contact Brandywine
Communications for more information on a particular time code.
•Serial I/O: The GPS4 features one bi-directional control port and one
dedicated output port used to provide time of day (TOD) information. The
TOD output is an ASCII data string that is outputted each second shortly
after the 1 PPS. These serial ports may be factory configured for either
RS232 (bi-polar) or RS422 (differential) signal levels.

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2 Product Specifications
2.1 Power Requirements
CONNECTOR DESCRIPTION
Connector Type MR series locking type AMP 1-640508-0
Pin 1 Positive
Pin 2 Chassis ground
Connector Pin
Pin 3 Negative
2.1.1 Standard
STANDARD DESCRIPTION
Input Voltage/Current 15 VDC @ 1.4 A maximum and 0.5 A steady state
Isolation None
2.1.2 Power Option 1
OPTION 1 DESCRIPTION
Input Voltage/Current 18 - 36 VDC @ 1 - 0.5 A maximum and 0.5 - 0.3 A steady state
Isolation 500 VDC
2.1.3 Power Option 2
OPTION 2 DESCRIPTION
Input Voltage/Current 36 - 60 VDC @ 1 - 0.34 A maximum and 0.5 - 0.2 A steady state
Isolation Isolated, 500 VDC
2.2 Outputs
2.2.1 PPS
PPS DESCRIPTION
Rate 1 pulse per second
Drive Output TTL into 50 ohm load
Reference Rising edge referenced to UTC (USNO)
Width 1 ms
Connector BNC female
±150 ns to UTC after the stabilization period of 24 hours – static and
time locked
Accuracy
Standard deviation of 34 ns
Less than 8 us/8 hr after the learning periodHoldover
Can be influenced by the antenna delay compensation

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2.2.2 10 MHz
10 MHz DESCRIPTION
Frequency 10 MHz
Period 100 ns
Waveform Sinusoid (standard) or square (link selectable)
Drive Level 1 Vrms (minimum) into 50 ohm load
Harmonics < 30 dB
100 Hz -140 dBc/Hz
1000 Hz -150 dBc/Hz
Phase Noise
10 KHz -155 dBc/Hz
1 sec 1 x 10
-11
10 sec 1 x 10
-11
100 sec 8 x 10
-12
Stability (Short Term)
1000 sec 1 x 10
-11
Long term 5 x 10
-12
while locked over 24 hours Accuracy
Holdover 1 x 10
-10
average during 24 hours
2.2.3 Synthesizer
SYNTH OPTION 1 DESCRIPTION
Frequency 19.6608 MHz
Connector BNC female
Drive Level Single ended, HCMOS
Duty Cycle 50/50 ± 10%
Long term 5 x 10
-12
while locked over 24 hoursAccuracy
Holdover 1 x 10
-10
average during 24 hours
SYNTH OPTION 2 DESCRIPTION
Frequency 2.048 MHz
Connector BNC female
Drive Level Single ended, HCMOS
Duty Cycle 50/50 ± 10%
Long term 5 x 10
-12
while locked over 24 hoursAccuracy
Holdover 1 x 10
-10
average during 24 hours
SYNTH OPTION 3 DESCRIPTION
Frequency 1.544 MHz
Connector BNC female
Drive Level Single ended, HCMOS
Duty Cycle 50/50 ± 10%
Long term 5 x 10
-12
while locked over 24 hoursAccuracy
Holdover 1 x 10
-10
average during 24 hours

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2.2.4 Status Indicators
INDICATOR COLOR DESCRIPTION
POWER GREEN Indicates the system power is available
LOCKED GREEN Indicates the system time locked status
HOLDOVER YELLOW Indicates the system is in holdover operation
ALARM RED Indicates the system fault
2.3 Inputs
2.3.1 System Reset
SYSTEM RESET DESCRIPTION
Function Resets the system microprocessor
Level +5 V, active high (pin 4)
Pulse Width 2 ms minimum
Sink Current 30 mA maximum
2.3.2 Antenna
ANTENNA DESCRIPTION
Connector BNC female
Impedance 50 ohms
Bias Voltage 5 VDC
Bias Current 80 mA maximum
2.4 Environmental
ENVIRONMENTAL DESCRIPTION
Operating -10°C to +60°C
Maximum Rate Change 15°C per hour
2.5 GPS Receiver
GPS RECEIVER DESCRIPTION
Number of Channels 8 parallel
Band L1 (1575.42 MHz)
2.6 Serial Control
SERIAL CONTROL DESCRIPTION
Connector DE-9 female

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2.6.1 Control Port
CONTROL PORT DESCRIPTION
75 – 19200 baud selectable by the DIP switchBaud Rate
4800, N, 8, and 1 is factory setting
2.6.2 TOD Port
TOD PORT DESCRIPTION
Protocol Time of day (ASCII)
Baud Rate 19.2 K
2.6.3 Serial Option 1
SERIAL OPTION 1 DESCRIPTION
I/O Level RS232
2.6.4 Serial Option 2
SERIAL OPTION 2 DESCRIPTION
I/O Level RS422
2.7 Hardware Status
HARDWARE DESCRIPTION
Connector DB-9
Level Open collector – pulled to ground through 100 ohm resistor
Maximum Pull-Up Voltage 5 VDC
Maximum Current 30 mA
Status Time lock (pin 8)
Status Fault (pin 9)
2.8 Physical
PHYSICAL DESCRIPTION
Length 5.84 inches (148.3 mm)
Width 5.58 inches (141.7 mm)
Height 1.61 inches (40.9 mm)
Weight 1.5 lbs (0.7 kg)

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2.9 Connectors and Indicators
The GPS4 provides access to the power, I/O, and signal connectors on the front
panel of the chassis. These various connectors and indicators are described
below.
LABEL FUNCTION DETAIL
PWR Power input connector Pins are numbered from left to right so
Pin 1 = VDC, Pin 2 = chassis, and Pin 3 =
ground
PIN RS232
(STANDARD) RS422 (OPTIONAL)
1 No connection Control TX-Y
2 Control TX Control TX-Z
3 Control RX Control RX-A
4 Hardware reset Control RX-B
5 Ground Ground
6 TOD TX TOD TX-Y
7 No connection TOD TX-Z
8 Time locked Time locked
I/O I/O connector provides I/O
for serial communications
and hardware status
indicators. Hardware
statuses are open collector
transistors that pull low
when the described status
is true.
9 Fault Fault
LABEL FUNCTION DETAIL
POWER Power good indicator LED Green LED indicates that +5 V of power is
present.
LOCKED Time locked indicator LED Green LED indicates time and position data is
synchronized since power up. Turns off after 8
hours of no satellite reception.
HOLDOVER Holdover indicator LED Yellow LED indicates the unit is in holdover.
ALARM Alarm indicator LED Red LED indicates:
- Phase lock loop control voltage is within 10%
of its limit.
- Synthesizer lost lock.
- Processor malfunctions.
- 8 hours of no satellite reception.
PPS 1 PPS output, BNC female Output of 1 PPS at TTL levels into a 50 ohm
load. 1 ms wide and rising edge on time.
10MHz 10 MHz sine wave output,
BNC female
Output of 10 MHz at 1 Vrms (2.8 Vp-p)
minimum into a 50 ohm load.
SYNTH Synthesizer output, BNC
female
Output of synthesizer frequency, HCMOS
levels. Frequency is programmed at the factory.
CODE Time code output, BNC
female
IRIG B/AFNOR time code:
- Modulated 3 Vp-p and modulation ratio 3:1
nominal (LK1 2-3).
- IRIG B DC level shift.
- TTL level into 50 ohm load (LK1 1-2).
ANT GPS antenna input, BNC
female
The input connects to a L1 band (1575.42 MHz)
GPS antenna. The “input” provides a +5 VDC
bias voltage to the GPS antenna at up to 80 mA
of current to operate a low noise amplifier or line
amplifier.

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3 Installation
To maximize the performance of the GPS4, please consider the following
guidelines .
3.1 Thermal Considerations
This unit is designed to operate over a temperature range of -10°C to +60°C, but
optimum performance will be achieved when the unit is allowed to operate at a
relatively constant temperature. The internal temperature rise is about 15°C
above the ambient temperature. For operation in environments above 50°C,
forced air cooling should be provided across the unit.
Power consumption will also vary as a result of temperature. This unit utilizes an
Ovenized Quartz Crystal Oscillator (OCXO) therefore a significant percent of the
overall power is used by the OCXO to maintain the thermal stability of the quartz
crystal oscillator. The OCXO achieves its frequency stability over temperature by
housing the quartz crystal and other thermally sensitive components in an “oven”.
The temperature of this “oven” is regulated and maintained at a relatively high
temperature usually 80°C to 90°C to a very close tolerance that is better than
0.1°C. This elevated temperature allows the oscillator to maintain good stability
while the ambient temperature is exposed to changes. At low ambient
temperatures the “oven” must generate and consume more power to maintain
the “oven” temperature. Higher ambient temperatures reduce the amount of
power needed to maintain the “oven” temperature.

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3.2 Power Supply Connections
The GPS4 has different power supply options made available. Regardless of the
power supply option installed, the power input to the unit is only DC and supplied
to the unit through a three pin power connector. This connector is labeled ‘PWR’
and is located in the upper left corner of the front panel.
The power connector pin assignments are shown below. Note that the power
connector pin assignments are the same for all power supply options. Pin 1 is
always the positive supply voltage, Pin 2 is always tied to the unit’s chassis
ground, and Pin 3 is always the negative supply voltage, also known as the
power return.
In all cases the maximum power required by the GPS4 is 20 watts. The
maximum power consumption will be experienced when power is first applied to
the unit and/or when the unit is operating at low temperatures (0°C to -10°C).
After approximately 10 minutes of continuous operation the start up power will be
reduced to approximately 6 to 8 watts when at room temperature (25°C).
Noise and ripple presence on the DC supply should be minimized. Certain
power supply options provide better noise and ripple rejection.
1 2 3
POS CHASSIS NEG
GND

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3.3 Unit Mounting
Four 4–40 threaded inserts are provided in the bottom of the unit for mounting
purposes. When mounting the unit to the interior of a cabinet or chassis some
consideration must be given to location from the cooling, vibration, and
electromagnetic interaction standpoint. One source of electromagnetic
interaction can be traced to the transformers used in the power supplies. Power
supplies that utilize toroidal wound transformers generally provide better
magnetic flux containment than the bobbin or laminated core type. In general,
keep the GPS4 away from the system power supply, particularly if the power
supply is AC-DC linear or switching type.
In most cases the GPS4 will be used in stationary installations where vibration
caused by dynamic movement will not be of any concern. However, the GPS4
utilizes a precision quartz oscillator and mechanical shock or vibration imparted
to the quartz crystal can generate unwanted electrical output in the form of
phase/frequency noise perturbations. In general, the GPS4 should be kept away
from sources of moderate or high vibration. In most cases, exposure to vibration
will not be of major concern and the degradation if any will not be noticeable.
The orientation of the unit should allow normal convection cooling. This means
that airspace must be made available around the perimeter of the air slots so that
natural airflow occurs. The unit should not be mounted upside down because
this will disrupt the convection airflow cooling, unless some form of forced air
cooling is provided. For operation in ambient temperatures at or above 50°C,
forced air cooling should be provided. Refer to section 3.1 (Thermal
Considerations).
The mounting screw length must not exceed 0.25 inches when measured from
the bottom surface of the unit.

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3.4 Serial Interface Connections
The GPS4 is shipped from the factory with either an RS232 or RS422 serial
interface (determined by user). A bi-directional interface known as the
‘TxD1/RxD1’ port and uni-directional serial output known as ‘TxD2/RxD2’ used to
output the time of day (TOD) are present on the DB-9 connector labeled ‘I/O’.
Both are simple interfaces that do not require or supply hand-shake signals.
Other signals present on the DB-9 connector are the ‘Time Locked’, ‘Fault’, and
‘HWRESET’ signals. These three signals provide open collector outputs used as
a hardware status indicator that reports the basic usability of the GPS4 and as a
system reset. It is not necessary to use these signals so these signals may be
left disconnected. The table below describes the pin assignments for both the
‘PWR’ (power) and ‘I/O’ connectors.
LABEL FUNCTION DETAIL
PWR Power input connector - Pins are numbered from left to right so
Pin 1 = VDC, Pin 2 = Chassis, and Pin 3 =
Ground.
- See the product label to determine the unit
supply voltage.
PIN RS232
(STANDARD) RS422 (OPTIONAL)
1 No connection TxD1-Y
2 TxD1 TxD1-Z
3 RxD1 RxD1-A
4 Hardware reset RxD1-B
5 Ground Ground
6 TxD2 TxD2-Y
7 No connection TxD2-Z
8 Time locked Time locked
I/O I/O connector provides I/O
for serial communications
and hardware status
indicators. Hardware
statuses (Pin 8 & 9) are
open collector transistors
that pull low when the
described status is true.
9 Fault Fault
3.5 Interfacing to the Serial Port
3.5.1 RS232
The connections necessary to interface the GPS4 control port to a standard PC
serial port are shown below.
2 2
3 3
5 5
PC Serial Port GPS4 Control Port

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3.5.2 TxD2 ASCII Time of Day Output
The time of day (TOD) output is made available on pin 6. This is an ASCII
message that is outputted every second and indicates the time of the last 1 PPS
output. Three formats are made available for this message.
3.5.3 RS422
The GPS4 may also use RS422 I/O levels. The RS422 interface uses balanced
(differential) wire pairs to transmit and receive data, thus four wires and a
common ground are required for this bi-directional interface. Two wires and a
common ground are required for a uni-directional interface such as the TOD
output. Since RS422 has more varied connector types and interfaces, no
specific connections are shown for this interface. Please refer to the pin
connections and signals listed in the table above.
3.6 Antenna Installation
3.6.1 Location
Several factors need to be considered when installing the GPS antenna. In most
cases the antenna is mounted externally (outdoor) and is exposed to the
elements. A good quality coaxial cable of 50 ohm impedance is required to
connect the GPS antenna to the GPS4. The cable provides two functions which
are to conduct the GPS RF signals (1575.42 MHz) that are received from the
GPS antenna to the GPS4 and to conduct the DC bias voltage (5 VDC) provided
by the GPS4 to the LNA (low noise amplifier) contained inside of the GPS
antenna. The antenna should be mounted securely, have a clear view of the sky,
and the top of the antenna should point upwards. In some installations it may not
be possible to mount the antenna such that it has a clear 360 degree view of the
sky. In such cases pick the location that affords the best view of the sky.
3.6.2 Exposure to High RF Fields
Some installations may occur in locations where a variety of high power
transmitters and antennas are present. The GPS antenna should not be directly
exposed to or bombarded with high level RF energy. In such cases locate the
antenna either above, below, or to the side of the high power RF transmission
antennas.

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3.6.3 Lightning Protection
The GPS4 does not provide any inherent protection against lightning strikes. In
general, lightning protection (when desired or needed) is provided by an
externally mounted protection device that is designed to shunt the high voltage
transient to a well established earth ground. Lightning arresting devices
designed for use in the GPS antenna systems are available at Brandywine
Communications (P/N 001000914).
3.6.4 Antenna Cable Factors
Other factors affecting antenna mounting location deal with the cable length
required to provide connection between the antenna and GPS4.
3.6.5 RF Loss
The RF signal attenuation experienced in the cable is important. The amount of
attenuation is related to the type (quality) of coaxial cable and cable length. The
antenna provides about 30 dB of gain to the received GPS signal. The purpose
of this gain is to offset the loss that is experienced in the cable between the GPS
antenna and GPS4. It is recommended that the overall antenna system gain
(antenna gain – cable loss) be between 10 dB and 33 dB. Thus, using an
antenna with 30 dB of gain allows for about 20 dB of cable loss. The GPS4 is
shipped with 100’ of antenna cable. Additional cable configurations are possible.
For distances beyond 330’ an in-line amplifier is required.
PART NUMBER CABLE LENGTH CABLE TYPE
002-0037 100 feet RG58 (standard)
002-0039 330 feet RG8
002-0040 150 feet RG8
051000001 In-line amplifier 20 dB TNC/TNC connectors

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4 Operating Instructions
4.1 Scope of Section
Section 4 covers the operation, initialization, and configuration of the GPS4. It is
assumed that power has been applied to the unit.
4.2 Operation
During normal operation the GPS4 automatically receives, analyzes, and
processes data from up to eight GPS satellites simultaneously. This data is used
to provide time, frequency, and position information, which are made available to
the user via a range of output interfaces.
In applications where the unit is to be used for general test and measurement
purposes it can be switched on and off as necessary. However, if the unit is to
be used as a time and/or frequency standard it should continuously run to allow
long term software processing to occur, which is essential if the unit is to achieve
its optimum performance. This process takes a minimum of 24 hours. The unit
carries out automatic position averaging and automatically transitions into this
‘known position’ to obtain the highest possible time and frequency precision. The
position averaging process takes place during the first 24 hours after switch on.
Whenever the GPS4 is powered on it carries out a self-test routine before
entering the operational program which sets the correct initial conditions for the
receiver system. Satellite tracking is automatic, provided that at least one is
visible and the GPS4 already has a valid position in its battery backed receiver.
The time and frequency control can start within approximately 1 minute. If three
or more satellites are visible, 3-D position information will also be processed.
The GPS4 internal frequency standard will warm up within 5 minutes. At the end
of the warm up period the frequency controller is reinitialized.
Synchronization of the local time to UTC (up to 15 minutes from satellite
reception) is confirmed by the illumination of the ‘LOCKED’ indicator. The
voltage control of the internal oscillator maintains synchronization of all outputs
with GPS. If the control voltage of the internal oscillator is within 10% of its end
of range, the red ‘ALARM’ indicator illuminates.

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The final phase and frequency locking process may take several hours
depending on the oscillator type fitted and satellite reception. The GPS4 uses a
patented "Intelligent Phase Locked Loop" system incorporating a specially
developed dynamic time constant, therefore the control parameters are
continuously modified to achieve lock in the shortest possible time and thereafter
optimize performance for the best time and frequency accuracy.
After a position has been successfully navigated the number of tracked satellites
drops to 4 or less, the satellite receiver automatically selects 2-D positioning
mode, and continues to recover time. If the number of tracked satellites then
drops to 3, 2, or 1 satellite positioning is suspended, but time is recovered. If no
satellites are tracked, the GPS4 runs in ‘HOLDOVER’ continuing to maintain all
outputs. After 8 hours of lost reception the red ‘ALARM’ indicator is turned on.
If the unit has a current satellite ephemeris at switch on, the start up time (after
system initialization has been completed) is approximately as follows:
•Initial acquisition : 1 - 15 seconds
•Satellite re-acquisition: 25 seconds
•Tracking: 20 seconds
•Navigating: 1 minute from start up
•Position averaging complete: 24 hours from start up
Synchronization of the receiver timing will normally be completed within 1½
minutes after switch on. If the receiver does not have an almanac already
backed up in memory after switch on, it will take typically 8 or 9 minutes to
acquire the necessary parameters to enable precise time acquisition indicated by
the illumination of the ‘LOCKED’ indicator on the GPS4 front panel.
For fixed position operation precise averaging of the position is carried out by the
GPS4 over 24 hours. The reason for this averaging is illustrated below. The
illustration shows that the raw time data before processing is carried out by the
GPS4 is improved by a factor of 3 when the GPS4 has averaged its position.
The vertical scale on the graphs is the 1 PPS deviation in nanoseconds.

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4.3 Serial Data Communication
The GPS4 has two asynchronous serial ports that communicate with the user.
These serial ports may be connected to other units such as printers, computers,
and terminals. These serial ports can be factory configured to be RS232 or
RS422. This section describes the options in more detail.
4.4 Control TxD1/RxD1
TxD1 is designed for communication with the user giving the user access to the
unit status and GPS reception. RxD1 receives command and data requests from
the user.
SW1 controls TxD1/RxD1 baud rate and character format. Prior to the switch
alteration the user must disconnect the power connection to the GPS4 and
observe all normal safety precautions when removing the cover to access the 8
way DIL switch (SW1).
SWITCH 1 2 3 4 5 6 7 8
ON - Baud Rate Selection -
See Table Below
Parity Odd Parity 8 Data
Bits
2 Stop
Bits
OFF No Parity Even
Parity
7 Data
Bits
1 Stop Bit
Table 3a TxD1/RxD1 Character Format
SW1-1 SW1-2 SW1-3 SW1-4 BAUD
OFF OFF OFF OFF 75
ON OFF OFF OFF 110
OFF ON OFF OFF 134.5
ON ON OFF OFF 150
OFF OFF ON OFF 300
ON OFF ON OFF 600
OFF ON ON OFF 200
ON ON ON OFF 1800
OFF OFF OFF ON 2400
ON OFF OFF ON 4800
OFF ON OFF ON 4800
ON ON OFF ON 9600
OFF OFF ON ON 9600
ON OFF ON ON 19200
OFF ON ON ON 19200
ON ON ON ON 19200
Table 3b TxD1/RxD1 Baud Rate