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| 型号： | GP2000 |
| 厂商： | Mitel Networks Corporation |
| 英文描述： | () |
| 中文描述： | （） |
| 文件页数： | 14/25页 |
| 文件大小： | 256K |
| 代理商： | GP2000 |
21
GP2000 – GPS CHIPSET DESIGNER’S GUIDE
Performance Parameters
SV selection strategy
The SV selection strategy (i.e. which satellites the receiver tries
to acquire) can be complex, and not always successful. If the number
of receiver channels is less than the number of visible satellites (or
satellites above the receiver elevation mask) then decisions have to
be made about which satellites to acquire.
A common criterion for selection is those satellites which give
the smallest dilution of precision in the navigation fix. This is fine
if the receiver antenna has full sky visibility but if there are ob-
structions shielding the satellites from the antenna (particularly
relevant in dynamic applications) then either some complex
software is required to swap satellites in and out of the acquisi-
tion loop or worse still, the receiver may stick to the ‘optimum’
constellation until either the satellites eventually become visible
or the best constellation changes. The number of satellites above
the horizon is obviously a position and time dependent quantity.
However, for reasonable latitudes (say below 660
°), the aver-
age number of visible satellites will be about 8 and on occasions will
increase to 10, 11 or even 12. Hence, having as many receiver
channels as visible satellites means that all satellites can be tracked
(or at least acquisition can be attempted) without the need for a
complex or time consuming selection strategy.
In GPS Architect, when sufficient data is available, the initial
satellite-to-channel allocation is based in descending elevation
angle and this is updated as satellites rise and set. The ability to
track all visible satellites (‘all-in-view’) also adds to the position
accuracy and availability.
Signal Acquisition Times
The initial signal acquistion time for a particular satellite in
GPS Architect is determined by how many frequency bins have
to be searched to find the signal. The number of bins is deter-
mined by the bin width and the maximum frequency excursions
due to the satellite receiver relative motion and the receiver clock
error. For a static receiver the maximum Doppler excursion is
about 65kHz.
The reference oscillator used with GPS Architect is a Rakon
TXO4010 with the following stability:
Temperature :
6
25 ppm (230 to 175
°C)
Supply voltage : 602 ppm (15V65%)
Ageing :
6
10 ppm/year
Assuming a total reference oscillator error of 64 ppm i.e.
6
63kHz at L1 plus the Doppler error of 650kHz gives a total
frequency excursion of about 6113kHz.
For the GPS Architect frequency bin width of 500Hz a total of 45
bins exists. For a 4-second bin search time this gives a total of 180
seconds to cover the complete range. However, if the clock error is
known (but no estimate of receiver or satellite positions) then this
decreases the search window to 650kHz, the number of bins to 20
and the search time to 80 seconds (worst case).
If the satellite almanacs, the reference clock error and a reason-
able estimate of the receiver position and current time (errors of
13Hz per km of position error and 09Hz per second of time error)
are available then it should be possible to constrain the search within
1 to 3 bins, giving an acquisition time of 4 to 12 seconds (Hot Start).
Bit And Frame Sync Times
Following signal acquisition, the next step is bit and frame sync.
A minimum of 2 seconds dwell time is required for bit sync to the
data transitions. To achieve frame sync the TLM and HOW words
need to be successfully received and the 20ms Epoch Counter set.
If valid data is being logged (continuous reception and no
parity errors), then TLM and HOW reception will take between
12 and 72 seconds. If the 20ms Epoch Counter is slewed then
frame sync will be delayed 6 seconds.
Hence, to go from code/carrier lock to bit and frame sync will
normally take between about 3 and 15 seconds.
Signal Re-Acquisition Times
When a signal is lost, the tracking loops for the channel concerned
go into a coast mode. In this mode the code and carrier DCOs are
left free running at their current values for the duration of the maximum
coasting interval (user configurable) or until the code and carrier
detection thresholds are exceeded. If the maximum coasting interval
is exceeded, the channel returns to the search mode. If the signal
reappears (e.g., after being temporarily obstructed by a building)
within the maximum coasting interval then signal reacquisition
typically occurs within 1 second.
Time-To-First-Fix
TTFF is dominated by how much initial information GPS
Architect has. Note: The satellite almanacs and ephemerides
can be read from file and the clock frequency error and initial
position estimate supplied via command line.
If no information is available (i.e., position estimate, time
estimate, reference oscillator error, satellite almanacs and
ephemerides) then a sky search occurs, selecting satellites
sequentially from their PRN ordering. In this scenario, TTFF is
typically 3 to 6 minutes* (Cold Start).
If GPS Architect has satellite almanacs, a reasonable time and
position estimate and a reference oscillator error estimate (the usual
scenario) then TTFF is typically less than 1 minute.* Note that it can
take 30 seconds to receive an ephemeris (Warm Start).
If GPS Architect has the above information and valid (less
than 4 hours old) satellite ephemerides then TTFF is typically
less than 30 seconds* (Hot Start).
*These figures assume a static receiver and unobstructed sky visibility.
Cold start
4ppm
Temperature, voltage
and ageing
Unknown
Condition
Reference oscillator error
Satellite almanacs
Satellite ephemerides
Initial position estimate
Initial time estimate
Warm start
Known to within 601ppm
1 week old
Unknown
< 100km
< 5 minutes
Hot start
Known to within 601ppm
N/A
< 4 hours old
< 100km
< 5 minutes
Table 2 Additional information on TTFF conditions
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