ETTC'2003 - SEE

arrival. The coordinates of four ground stations are known, and then the position of the reentry vehicle
can be computed. For the 3∆t system working in passive mode, it can make use of the existing telemetry
system. No more equipment is needed in reentry vehicle. Only one time-recorder is needed in each of
ground stations. It is one of its advantages that the system is simple.
PRINCIPLE OF 3∆t SYSTEM
Fig 1. Shows the composition of 3∆t passive locating system. The ground stations receive the
GPS Satellite
ground stations
reentry vehicle
Fig 1. the 3Δt passive locating system
common telemetry signal sent from reentry vehicle, and record the time of arrival respectively, then
r1
t1
= T0
+
c
r2
t2
= T0
+
c
(1)
M
t
N
rN
= T0
+
c
where ( i 1,
2,
L,
N)
is the time of arrival, T is the time of signal sent, ( i = 1,
2,
L,
N)
is the
t i
distance between ground stations and reentry vehicle respectively, c is the velocity of light in meters
per second.
= 0
Provided that the coordinates of ground station is
[ ] T
x , y , z
XT = T T T
, then
For the T0
is unknown, then
i [ ] T
x , y , z
Xi i i i
r −
r i
= , the coordinates of target is
2
2
2
2
i = ( xT
− xi
) + ( yT
− yi
) + ( zT
zi
)
(2)