TM 5-6675-318-14
b.
The satellite message is interpolated by the MX 1502 to define accurately the
s a t e l l i t e ’ s p o s i t i o n.
T o o b t a i n a p o s i t i o n f i x,
the MX 1502 measures its position
r e l a t i v e t o t h e k n o w n p o s i t i o n o f t h e s a t e l l i t e.
This is done by counting the
number of doppler cycles between each time mark.
A doppler count between two time
marks provides the change in range between the MX 1502 and satellite over the time
i n t e r v a l.
The doppler data from the two channels is combined to provide accurate
s l a n t r a n g e s t o t h e s a t e l l i t e.
Position fix is calculated by comparing the range
changes which occurred between the estimated position of the MX 1502 and the known
position of the satellite as measured by the doppler counts. The MX 1502 then
adjusts its internal position until the calculated range changes best match those
measured by the doppler counts.
A minimum of three parameters are determined in the
p o s i t i o n f i x c o m p u t a t i o n.
These parameters are latitude, longitude and doppler
frequency.
After the input data has been stored in the computer memory, the MX 1502
performs 10 steps to compute a position fix. The ten steps are:
Check satellite message for new message injection.
Majority vote redundant message data.
C o r r e c t d o p p l e r d a t a f o r r e f r a c t i o n.
C o m p u t e s a t e l l i t e p o s i t i o n s.
E s t i m a t e r e c e i v e r p o s i t i o n.
Calculate satellite-to-receiver ranges at the beginning and end of each
doppler count period differences derived.
Compare measured range from doppler counts with calculated range
d i f f e r e n c e s.
Adjust estimate of receiver position until calculated and measured range
d i f f e r e n c e s a g r e e.
C a l c u l a t e r e c e i v e r p o s i t i o n.
Record calculated position fix on cassette tape.
c .
If a new message is injected into the satellite memory while the MX 1502 is
r e c e i v i n g t h e s a t e l l i t e s i g n a l s , t w o d i f f e r e n t d e s c r i p t i o n s o f t h e s a m e o r b i t w i ll
be obtained.
Only one description can be used.
Therefore, the MX 1502 must check
for an injection and eliminate unwanted data.
The check is performed by a bit-by-
bit majority vote among the received satellite messages. This step eliminates
errors which may have occurred during receipt of the satellite message.
The
majority-voted message is used to calculate earth-fixed Cartesian coordinates for
the satellite at the beginning and end point of each doppler count collected during
the pass.
The doppler data from the two channels is combined to provide refraction-
corrected doppler information.
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