Mitel GPS Orion-S/-HD Receiver - Manual (Page 32)

Document Title: 28
User’s Manual for the GPS Orion-S/-HD Receiver
Document No. Issue 1.0
GTN-MAN-0110 June 22, 2003
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DLR/GSOCNo part of this document shall be reproduced in any form or disclosed to third parties without prior authorization.
4.3.3.2 DW – Doppler Window
If code lock for an allocated GPS satellite is not acquired within a given time, the search is
extended in neighbouring frequency bins at steps of ±500 Hz. The maximum Doppler offset
considered in this search can be specified by the DW command. Its value should account for
the expected reference frequency error (±1575 Hz at 1 ppm) and the uncertainty of the pre-
dicted Doppler shift. A peak line-of sight velocity of about 1000 m/s is encountered in terres-
trial applications, thus requiring an additional ±5 kHz window in cold start tracking mode. For
LEO satellites, a window of up to ±40 kHz may be required.
Execution of the command is acknowledged by an F98 command reply of the form
I-DW-Changed Doppler window to 11500Hz
Negative input values are ignored and an error message
E-DW-Ignored (invalid argument)
is issued.
CmdID Chars. Format Description
DW 72 Set Doppler window
1 x <STX>
2 xx Command Id (=DW)
5 xxxxx Doppler search interval [Hz] ( ±value)
2 xx Checksum
1 x <ETX>
Notes:
•
The specified search window is applied in cold start tracking mode or whenever a
navigation fix has not been achieved within the past two minutes. Otherwise a one-
bin search in the nominal Doppler window is carried out based on the available posi-
tion/velocity information and oscillator offset.
Example:
<STX>DW150000E<ETX>
Extend frequency search to ±15 kHz
<STX>DW00E<ETX> Enforce one-bin Doppler window (requires accurate a priori
navigation information and oscillator error knowledge)