Analyzing Signal-to-Noise Ratio
34
P/N: 1004636-01
System
Setup
Analyzing Signal-to-Noise Ratio
Knowing the strength and reliability of the ranging signal transmitted from the satellites will help determine the
quality of the satellite signals. Use Table 6-1 to help estimate signal quality from a satellite vehicle.
If the SNR value of a satellite signal is less than the threshold value from the table, then pay close attention to
this satellite because it can potentially cause problems for getting accurate timing and positioning results.
Collecting Almanacs and Ephemerides
Each satellite broadcasts a navigation message that includes the ephemeris parameters of the satellite, the
almanac, and various other information. The ephemeris parameters describe the orbital motion of the satellite
and are used to predict its location/trajectory. The almanac gives the approximate orbit (course) for the
transmitting satellite and all other satellites in the same system only.
•
GPS and GLONASS satellites broadcast ephemeris data cyclically within 30 seconds.
•
GPS satellites broadcast almanac data cyclically within 12.5 minutes; GLONASS satellites broadcast almanac
data cyclically within 2.5 minutes.
When the receiver has an almanac, you can considerably reduce the time needed to search for and lock onto
satellite signals.
The receiver regularly updates the almanac and ephemerides and stores the most recent versions in its Non-
Volatile Random Access Memory (NVRAM).
You need to collect or update the almanac and ephemerides under the following circumstances:
•
If the receiver has been off for a long time.
•
If the last known receiver position, stored in the NVRAM, is different from the present position by several
hundred kilometers.
•
After loading a new OAF.
•
After loading new firmware.
•
After clearing the NVRAM.
To collect almanacs and ephemerides:
1. Set up the receiver.
The external antenna should be in a location with a clear view of the sky.
2. Turn on the receiver.
3. Wait for about 15 minutes while the receiver collects complete almanac and ephemeris data from the
satellites.
Table 6-1. Typical SNR Values
SV
a
Elevation
(degrees)
a. SV = satellite vehicle
C/A channel
(dB*Hz)
P1 channel
(dB*Hz)
P2 channel
(dB*Hz)
10–20
>35
>10
>10
20–40
>40
>20
>20
40–60
>45
>30
>30
60–90
>50
>40
>40
If 15 minutes have passed and the receiver does not lock onto satellites, then clear the NVRAM. See
“Resetting the Receiver (Clearing the NVRAM)” on page 30 for details.
