The Palisade GPS receiver is an 8-channel smart antenna, housing the GPS
receiver, antenna and interface in a single unit, and is designed for rooftop
deployment in static timing applications.
Palisade generates a PPS synchronized to UTC within +/- 100 ns.
The Palisade's external event input with 40 nanosecond resolution is utilized
by the Palisade NTP driver for asynchronous precision time transfer.
No user initialization of the receiver is required. This driver is compatible
with the following versions of Palisade:
Version
Event Input
Trimble Part Number
7.02
No
26664-00
7.02E
Yes
26664-10
7.12
Yes
38158-00
Note: When using Palisade 26664-00, you must set fudge flag2 to 1 in
ntp.conf.
See configuration.
GPS Installation
A location with unobstructed view of the horizon is recommended. Palisade
is designed to be securely mounted atop standard 3/4 inch threaded pipe.
The 12 conductor (dia. 10 mm) power and I/O cable must be routed
from the rooftop site to the NTP server and properly strain relieved.
GPS Connection
The Palisade is equipped with dual (A & B) RS-422 serial interfaces
and a differential TTL PPS output. An RS-232 / RS-422 Interface Module
is supplied with the Palisade NTP Synchronization Kit. Palisade port
A must be connected to the NTP host server. Maximum antenna cable length
is 500 meters. See the pinouts table for detailed
connection Information.
Palisade's port B provides a TSIP (Trimble Standard
Interface Protocol) interface for diagnostics, configuration, and monitoring.
Port B and the PPS output are not currently used by the Palisade NTP reference
clock driver.
O/S Serial Port Configuration
The driver attempts to open the device /dev/palisadeu
where
u is the NTP refclock unit number as defined by the
LSB of the refclock address. Valid refclock unit numbers are 0 -
3.
The user is expected to provide a symbolic link 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, based on the refclock unit number, where unit 1
corresponds to COM1: and unit 3 corresponds to COM3:
NTP Configuration
Palisade NTP configuration file "ntp.conf" with event polling:
#------------------------------------------------------------------------------ # The Primary reference server 127.127.29.0 # Trimble Palisade GPS Refclock Unit #0 peer terrapin.csc.ncsu.edu # internet server # Drift file for expedient re-synchronization after downtime or
reboot. driftfile /etc/ntp.drift #------------------------------------------------------------------------------
Configuration without event polling:
#------------------------------------------------------------------------------ # The Primary reference server 127.127.29.0 # Trimble Palisade GPS (Stratum 1). # Set packet delay fudge 127.127.29.0 time1 0.020 # and set flag2 to turn off event polling. fudge 127.127.29.0 flag2 1 #------------------------------------------------------------------------------
Time Transfer and Polling
Time transfer to the NTP host is performed via the Palisade's comprehensive
time packet output. The time packets are output once per second, and whenever
an event timestamp is requested.
The driver requests an event time stamp at the end of each polling interval,
by pulsing the RTS (request to send) line on the serial port. The Palisade
GPS responds with a time stamped event packet.
Time stamps are reported by the Palisade with respect to UTC time. The
GPS receiver must download UTC offset information from GPS satellites.
After an initial UTC download, the receiver will always start with correct
UTC offset information.
Run NTP in Debugging Mode
The following procedure is recommended for installing and testing a Palisade
NTP driver:
Perform initial checkout procedures. Place the GPS receiver outdoors; with
clear view of the sky. Allow the receiver to obtain an UTC almanac.
Verify presence of timing packets by observing the 1 Hz (PPS) led on the
interface module. It should flash once per second.
Connect Palisade's port A to the NTP host.
Configure NTP and the serial I/O port on the host system.
Initially use fudge flag2 in ntp.conf,
to disable event polling (see configuration).
Run NTP in debug mode (-d -d), to observe Palisade_receive events.
Remove fudge flag2 and restart ntpd in debug mode to observe palisade_receive
events.
If event polling fails, verify the connections and
that the host hardware supports RTS control.
Event Logging
System and Event log entries are generated by NTP to report significant
system events. Administrators should monitor the system log to observe
NTP error messages. Log entries generated by the Palisade NTP reference
clock driver will be of the form:
Nov 14 16:16:21 terrapin ntpd[1127]: Palisade #0: message
Specifies the time offset calibration factor, in seconds and fraction,
with default 0.0. If event capture is not used, time1 should be set to
20 milliseconds to correct serial line and operating system delays incurred
in capturing time stamps from the synchronous packets.
stratum number
Specifies the driver stratum, in decimal from 0 to 15, with default 0.
When set to 1, driver does not use hardware event capture. The synchronous
packet output by the receiver at the beginning of each second is time stamped
by the driver. If triggering the event pulse fails, the driver falls back
to this mode automatically.
DEFINEs
The following constants are defined in the driver source code. These defines
may be modified to improve performance or adapt to new operating systems.
Label
Definition
Default Value
DEVICE
The serial port device to be used by the driver
/dev/palisadeu
PRECISION
Accuracy of time transfer
1 microsecond
CURRENT_UTC
Valid GPS - UTC offset
13
SPEED232
Host RS-232 baud rate
B9600
TRMB_MINPOLL
Minimum polling interval
5 (32 seconds)
TRMB_MAXPOLL
Maximum interval between polls
7 (128 seconds)
Data Format
Palisade port A can output two synchronous time packets. The NTP driver
can use either packet for synchronization. Packets are formatted as follows:
Packet 8F-AD (Primary NTP Packet)
Byte
Item
Type
Meaning
0
Sub-Packet ID
BYTE
Subcode 0xAD
1 - 2
Event Count
INTEGER
External event count recorded (0 = PPS)
3 - 10
Fractional Second
DOUBLE
Time elapsed in current second (s)
11
Hour
BYTE
Hour (0 - 23)
12
Minute
BYTE
Minute (0 - 59)
13
Second
BYTE
Second (0 - 59; 60 = leap)
14
Day
BYTE
Date (1 - 31)
15
Month
BYTE
Month (1 - 12)
16 - 17
Year
INTEGER
Year (4 digit)
18
Receiver Status
BYTE
Tracking Status
19
UTC Flags
BYTE
Leap Second Flags
20
Reserved
BYTE
Contains 0xFF
21
Reserved
BYTE
Contains 0xFF
Leap Second Flag Definition:
Bit 0: (1) UTC Time is available
Bits 1 - 3: Undefined
Bit 4: (1) Leap Scheduled: Leap second pending asserted by GPS
control segment.
Bit 5: (1) Leap Pending: set 24 hours before, until beginning
of leap second.
Bit 6: (1) GPS Leap Warning: 6 hours before until 6 hours after
leap event
Bit 7: (1) Leap In Progress. Only set during the leap second.
Tracking Status Flag Definitions:
Code
Meaning
Accuracy
Receiver Mode
0
Receiver is Navigating
+/- 1 us
Self Survey
1
Static 1 Sat. Timing Mode
+/- 1 us
1-D Timing
2
Approximate Time
20 - 50 ms
Acquisition
3
Startup
N/A
Initialization
4
Startup
N/A
Initialization
5
Dilution of Position too High
5 ppm
Self Survey
6
Static 1 Sat. Timing: Sat. not usable
5 ppm
1-D Timing
7
No Satellites Usable
N/A
Self Survey
8
Only 1 Satellite Usable
20 - 50 ms
Self Survey
9
Only 2 Satellite Usable
20 - 50 ms
Self Survey
10
Only 3 Satellites Usable
20 - 50 ms
Self Survey
11
Invalid Solution
N/A
Error
12
Differential Corrections
N/A
N/A
13
Overdetermined Fixes
+/- 100 ns
Timing Steady State
Packet 8F-0B (Comprehensive Timing Packet)
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
Note: If driving the RS-422 inputs on the Palisade single ended,
i.e. using the Green and Gray connections only, does not work on all serial
ports. Use of the Palisade NTP Synchronization Interface Module is recommended.
The 12 pin connector pinout definition:
Face the round 12 pin connector at the end of the cable, with the notch
turned upwards.
Pin 1 is to the left of the notch. Pins 2 - 8 wrap around the bottom,
counterclockwise to pin 9 on the right of the notch. Pin 10 is just below
the notch. Pins 10 (top), 11 (bottom left) and 12 (bottom right) form a
triangle in the center of the connector.
Pinouts for the Palisade NTP host adapter
(Trimble PN 37070) DB-25 M connector are as follows: