Trimble Palisade Receiver

Synopsis

Description

This documentation describes version V1.30 (May 5, 1998) of the source.

Compatibility

Please note that Windows NT deviates from Unix systems in the implementation of the select() system call. This shortcoming required extensive workaround. Reference clock support for Windows NT is currently not available with the NTP distribution from the University of Delaware.

Driver code for the Palisade receiver is being maintained by:
Sven Dietrich
Trimble Navigation Ltd.
645 North Mary Ave.
Sunnyvale, CA 94088-3642

Please send bug reports, comments and suggestions to the above addresses.

Specification

Receiver

Palisade generates a PPS synchronized to UTC within +/- 100 ns.  This level of precision is achieved using an over-determined time solution, a 40 ns pulse steering resolution and a T-RAIM (Time-Receiver Autonomous Integrity Monitor) algorithm.

During initialization the Palisade reports GPS time, which differs from UTC by an integer number of seconds corresponding to the number of leap seconds since 1980, the beginning of the GPS epoch.

The current GPS / UTC time offset was 12 seconds (as of May 1998). After the GPS / UTC offset has been obtained from GPS satellite data, the  receiver will report UTC time. Typically, UTC time will be available a few minutes after cold-start power-up. The NTP driver will discard time stamps received from the Palisade GPS until they contain valid UTC offset information.

Installation

The 12 conductor power and I/O cable must be routed from the rooftop site to the NTP server.

Connection

The following connectivity must be provided by serial cables or adapters:
 

Description	DB-9	DB-25
Transmit Data	2	3
Receive Data	3	2
Request to Send	7	4
Signal Ground	5	7

The Palisade's primary port, Port A, provides a TSIP (Trimble Standard Interface Protocol), control and command interface for diagnostics, configuration, and monitoring. Port A and the PPS output are not currently used by the Palisade NTP refclock driver.

O/S Serial Port Configuration

The device name is defined as a constant in the code, and the user is expected to provide a symbolic link from /dev/palisadeu to an available serial port device.  This is typically performed by a command such as:

ln -s /dev/ttyS0 /dev/palisade0

Windows NT does not support symbolic links to device files. COMx: is used by the driver, derived from the refclock unit number, where unit 0 corresponds to COM1: and unit 3 corresponds to COM4:

Initialization

During initialization, the driver calls the polling function several times in order to obtain an estimate of the code delay associated with execution of the poll sequence.  The driver attempts to compensate for code execution delays on the host processor.

Time Transfer and Polling

The driver requests an event time stamp at the end of each polling interval (32 s default), by pulsing the RTS (request to send) line on the serial port. The Palisade receiver responds with a time stamped event packet.

Time stamps are reported by the Palisade with respect to UTC time. The NTP host will handle daylight savings transitions.

Data Format

Port B outputs the synchronous time packet, which is formatted as follows:
 

Byte	Item	Type	Meaning
0	Sub-Packet ID	BYTE	Subcode 0x0B
1 - 2	Event Count	INTEGER	External event count recorded (0 = PPS)
3 - 10	UTC / GPS TOW	DOUBLE	UTC / GPS time of week (seconds)
11	Date	BYTE	Day of Month
12	Month	BYTE	Month of Event
13 - 14	Year	INT	Year of event
15	Receiver Mode	BYTE	Receiver operating dimensions:  0: Horizontal (2D)  1: Full Position (3D)  2: Single Satellite (0D)  3: Automatic (2D / 3D)  4: DGPS reference  5: Clock hold (2D)  6: Over determined Clock
15 - 17	UTC Offset	INTEGER	UTC Offset value (seconds)
18 - 25	Oscillator Bias	DOUBLE	Oscillator BIAS (meters)
26 - 33	Oscillator Drift Rate	DOUBLE	Oscillator Drift (meters / second)
34 - 37	Oscillator Bias Uncertainty	SINGLE	Oscillator bias uncertainty (meters)
38 - 41	Oscillator Drift  Uncertainty	SINGLE	Oscillator bias rate uncertainty  (meters / second)
42 - 49	Latitude	DOUBLE	Latitude in radians
50 - 57	Longitude	DOUBLE	Longitude in radians
58 - 65	Altitude	DOUBLE	Altitude above mean sea level, in meters
66 - 73	Satellite ID	BYTE	SV Id No. of tracked satellites

 

Configuration

# Fudge stratum and other features as required.
fudge 127.127.29.0 stratum 1

peer terrapin.csc.ncsu.edu # another very good, friendly clock

# Drift file for expedient re-synchronization after downtime or reboot.
driftfile /etc/ntp.drift
#------------------------------------------------------------------------------

A typical ntp.conf configuration file for this driver and a Palisade without event polling:

#------------------------------------------------------------------------------
# The Primary reference
server 127.127.29.0 # Trimble Palisade GPS (Stratum 1).

# Fudge stratum and other features as required
# Set serial port transmission + acknowledgment delay to a good average
# and set flag2 to indicate not using event polling.
fudge 127.127.29.0 stratum 1
fudge 127.127.29.0 time1 0.020
fudge 127.127.29.0 flag2 1

# Drift file for expedient re-synchronization after downtime or reboot.
driftfile /etc/ntp.drift
#------------------------------------------------------------------------------

Logging

Nov 14 16:16:21 terrapin ntpd[1127]: Palisade #0: fd: 8 dev: /dev/palisade0
Nov 14 16:16:21 terrapin ntpd[1127]: Palisade #0: code delay: 19 us 10 polls

Debugging

1. Place the Palisade GPS receiver outdoors; with clear view of the sky.
2. Verify functionality of the receiver and I/O interface, by observing automatic packet traffic on port A. Use TSIPCHAT or another TSIP decoder for decoding packet data. In a bind, a terminal program may be used to verify incoming binary data without decoding.
3. Allow the receiver to acquire satellites. The Palisade will not respond to event polls or output synchronous packets, when not tracking  satellites. NTP will report no-reply events if timestamps are not received. Excessive NO-REPLY events indicate reception or cabling problems. In debug mode, the driver will report the id of up to 8 satellites in view.
4. Connect Palisade port B to the NTP host.
5. Configure NTP and the serial I/O port on the host system.
6. Initially use fudge flag2 in ntp.conf, to disable event polling (see configuration).
7. Run NTP in debug mode (-d), observe Palisade_receive events.
8. Time stamp packets are only output while the receiver is tracking satellites.
9. Remove fudge flag2 and restart in debug mode to observe event polling receive events.

#DEFINEs

 

Label	Definition	Default Value
DEVICE	The serial port device to be used by the driver.	/dev/palisadeu
PRECISION	Estimate of the accuracy of timestamps when using event poll.	10 microseconds
POLL_AVG	Number of polls to execute when estimating processor code delay.	10 polls
CURRENT_UTC	Valid GPS - UTC offset	12
SPEED232	Host RS-232 baud rate	B9600
TRMB_MINPOLL	Minimum polling interval	5 (32 seconds)
TRMB_MAXPOLL	Maximum interval between polls	7 (128 seconds)

 

Fudge Factors

time1 time

Specifies the time offset calibration factor, in seconds and fraction, with default 0.0.

The driver attempts to determine the code path delay incurred when triggering event capture. When event capture is not being used, time1 should be set to approximately 10 milliseconds to offset serial line and operating system delays incurred in capturing time stamps from the synchronous packets.

 

time2 time

Not currently used by this driver.

 

stratum number

Specifies the driver stratum, in decimal from 0 to 15, with default 0.

It is suggested that the clock be fudged to stratum 1 so this it is used a backup time source rather than a primary when more accurate sources are available.

 

refid string

Specifies the driver reference identifier, GPS.

 

flag1 0 | 1

Not currently used by this driver.

 

flag2 0 | 1

If set to 1, the driver does not use hardware event polling, but instead listens for synchronous packets that are output at the start of each second by the Palisade receiver. If the ioctl() call responsible for triggering the event capture fails, the driver falls back to

this mode automatically.
 

flag3 0 | 1

Not currently used by this driver.

 

flag4 0 | 1

Not currently used by this driver.

Additional Information