User's Manual

DEC. 2011—EFT REVIEW DRAFT—CISCO CONFIDENTIAL
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Cisco 1240 Connected Grid Router Hardware Installation Guide
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Chapter 2 Router Hardware Description
Hardware Features Detailed Description
• Monitor mode—The GPS operates in monitor mode when you upgrade the firmware that is stored
in the
SD Flash Memory Module.
• Standby mode—When the router AC power supply fails or is not present, and the battery backup
unit is providing power to the router, the GPS operates in standby mode. The GPS receiver is
disabled but the GPS RAM and the real-time clock remain active. In this mode, the GPS RAM stores
the GPS almanac, ephemeris, and last position.
– When the GPS is in standby mode for less than two hours, it performs a hot start when normal
router power is restored.
– When the GPS is in standby mode for more than two hours, it performs a warm start when
normal router power is restored.
GPS LED
You can view the GPS LED to determine the GPS state and whether or not it is successfully connected
to a GPS satellite. For information on the GPS LED, see the chapter
Router LED Locations and States.
GPS Timing Messages
GPS positioning messages contain a timestamp which can be up to two seconds in the past, so the router
uses data contained in timing messages described
Table 2-12 as the source of time for the GPS.
Note The GPS time must calculate in the Universal Coordinated Time (UTC) offset, which is used to set the
local time.
Acquiring the Correct Time
Take the following steps to ensure the GPS acquires an accurate time:
Step 1 To eliminate UTC offset, confirm that the almanac is complete and the receiver is generating 3D fixes.
Step 2 Confirm that the GPS receiver is configured for the late PPS (Pulse-Per-Second) option (the receiver
outputs one PPS for a 3D fix).
Step 3 Capture the time from TSIP packet 0x41 or TSIP packet 0x8F-20.
Step 4 On the next PPS, add 1 to the whole second captured in Step 3 to adjust to the correct time.
Table 2-12 GPS Timing Protocols and Messages
Protocol Name Message Type Containing Time Information
TSIP (Trimble Standard Interface Protocol) Packets 41 and 8F-21.
TAIP (Trimble ASCII Interface Protocol) TM messages
NMEA (National Marine Electronics Association) ZDA message.