TM 5-6675-318-14
4-3. TECHNICAL PRINCIPLES OF OPERATION.
4 - 3 . 1 G e n e r a l .
The MX 1502 tracks the 150 and 400 MHz, phase-modulated signals
radiated by orbiting satellites of the TRANSIT satellite system.
The frequencies
transmitted are doppler shifted and contain satellite data. The use of two
frequencies is to minimize the effects of ionospheric interference with the doppler
s h i f t .
When both frequencies are received by the MX 1502, the computer programming
will compensate for ionospheric effects using doppler shift. Using the doppler
shift and satellite position data the MX 1502 calculates the position of the antenna
r e f e r e n c e p o i n t .
The use of two frequencies sharply increases position fixing
accuracy.
Following the first usable satellite pass at each new site the MX 1502 can
display a 2D position with precision that is accurate enough to verify that the
l o c a t i o n i s , i n f a c t , t h a t o f t h e d e s i r e d s i t e .
After three or four acceptable
satellite passes, the MX 1502 will have acquired enough data to display a 3D point
position. Quality indices may also be displayed to determine if sufficient data has
been collected in relation to the survey accuracy requirements. The 3D position is
a cumulation of all acceptable satellite passes for the site.
D u r i n g a s a t e l l i te
pass, all data collected is stored in memory until the pass is completed. A 2D and
3D position is then computed and stored in memory and on magnetic tape cassette.
The tape cassette can then be used for post-processing of the site position or for
t r a n s l o c a t i o n p r o c e s s i n g.
Real time translocation processing can also be computed
if an external communications link is utilized from a known site to a remote,
unknown site.
Positions calculated by the MX 1502 are in satellite datum. The
reference ellipsoid used is the same as WGS-72 datum.
Automatic conversion to local
datum, XYZ (Cartesian), and universal transverse mercator (UTM) can be accomplished
by entering datum shift constants and commanding display of the datum shifted
coordinates .
The transit satellite system was developed by the U.S. Government to support the
U.S. Navy submarine force for the inertial navigation equipment aboard polaris
submarines and became operational in January, 1964.
T h e s a t e l l i t e s c i r c l e t h e e a r t h
in 107 minute polar orbits at an altitude of approximately 1000 kilometers .
These
orbits are wide at the equator and converge at the poles. Although the orbits are
f r o m p o l e - t o - p o l e , t h e s a t e l l i t e s d o n o t r o t a t e w i t h t h e e a r t h . I f t h e o r b i t s c o u l d
be traced out, they would suggest a spherical bird cage inside of which the earth
r o t a t e s o n i t s a x i s.
Every point of the earth will thus pass under each orbit
approximately twice a day.
W i t h f i v e o p e r a t i o n a l s a t e l l i t e s , t h i s w i l l p r o v i d e an
acceptable satellite every 35 to 100 minutes.
T h e t r a n s i t s a t e l l i t e s y s t e m is
managed and monitored by the Naval Astronomical Group based at Point Mugu,
California, with two additional tracking stations located in Hawaii and Minnesota.
Data in each satellite is updated on a daily basis by the controlling authority.
a.
Each satellite continuously transmits a message that lasts exactly two
minutes beginning and ending at each even minute.
Because this message is precisely
controlled, it serves as an accurate even minute mark. The satellite message
contains several different segments.
The segments used by the MX 1502 are:
S a t e l l i t e p o s i t i o n
J u l i a n d a te
Universal time (GMT)
Doppler cycles (23 seconds long)
4-9
