The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part Bl. Beijing 2008 
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Clearly, errors in knowledge of the quantities in this equation 
impact the total INSAR performance. The propagation of a 
baseline uncertainty to a topographic height error has already 
been derived by Zebker and Goldstein (1986). The high 
sensitivity of the InSAR height on B, a and O puts 
extremely high requirements on the knowledge of these 
parameters. The trade-off between these errors was a large part 
of the mission design. Supposing effective across-track 
baselines B for high resolution DEM’s is in the order of 1 km, 
Table 2 represents the error in elevation caused by errors of the 
INSAR system. 
3. GPS APPLICATION 
Dual frequency GPS are capable of eliminating the ionospheric 
signal errors and thus to make best use of the high accuracy 
GPS carrier phase measurements. Aside from space 
applications like real-time positioning, precise orbit 
determination, attitude determination of spacebome sensors, 
dual-frequency GPS receivers are considered as practical and 
cost-effective system for precise baseline determination and 
time & frequency synchronization in DSS missions. 
3.1 Position Determination 
INSAR accuracy is obtained by careful measurement of the 
baseline length and orientation and the location of the platform 
relative to the reference coordinate system. 
3.1.1 Baseline Estimation 
In INSAR processing the knowledge of precise geometrical 
parameters of a DSS is essential. The precise baseline vector 
determination is based on direct evaluation of dual frequency 
GPS carrier phase measurements. Recent studies have 
demonstrated the usefulness of GPS receivers for relative 
positioning of formation flying satellites using dual-frequency 
carrier-phase observations(Meyer,2004). The analysis 
performed with GFZ's Earth Parameter and Orbit System 
utilizing an adapted GRACE configuration shows that a 
relative position knowledge of 1 mm can be achieved in most 
cases( Kroes,2005). 
The measured position is that of the GPS antenna. Considering 
the impact of satellite attitude errors and uncertainties in both 
the GPS and the SAR antenna phase centre positions, Baseline 
estimation could be realized with 8mm accuracy(three axis). 
3.1.2 Precise Orbit Determination 
Precise knowledge of the orbit is required in order to obtain the 
required 1-m position determination. Furthermore, formation 
flying DSS also require an accurate real-time knowledge of the 
position. In accordance with INSAR applications, dual 
frequency GPS receivers are also preferred for precise orbit 
determination and navigation of LEO spacecraft. Tracking 
Accuracy can reach: < 10m (real time); < 10 cm RMS (post- 
processed); velocity accuracy can reach:< 10cm/sec (real-time). 
3.2 Time & Frequency Synchronization 
Time & frequency synchronization error occurs because of the 
different formation flying platforms and different frequency 
sources. The phase coherence of different echoes, which is the 
key to SAR imaging and INSAR processing, must be 
guaranteed. Hence there must be higher requirement of the 
accuracy for time and frequency synchronization. For a DSS 
operation, some means must be provided to ensure that signals 
are received at the proper time and frequency. 
3.2.1 Frequecny synchronization 
Frequency synchronization is required to position the receive- 
only signal within the azimuth prefilter bandwidth, also the 
range shift of the impulse response will be dominated by 
deviations between the PRFs of the transmitter and receiver. 
Since the PRF is usually derived from the STALOs , the shift 
in slant range may be derived as 
2.M0 = _l^_ (6) 
ct 10 c-/ 
where r is the range resolution, c is the light velocity, t is the 
allowable payload working duration, y is the relative frequency 
deviation between the two STALOs . By solving formula (11), 
we get y<5E-l 1. 
All required transmitter and receiver frequencies are generated 
from a stable local oscillator (STALO) by means of 
multiplication, division and phase-locked loops. The internal 
STALO with 5 x 10-Ell short term stability determines the 
short time stability which is required by master satellite SAR 
prcessing, while the long time stability is disciplined to the 
stability of the GPS satellites atomic clocks. The atomic clock 
onboard the GPS satellite has an accuracy of Af/f <10-14. As 
long as the receiver is locked to the GPS, the long time stability 
of STALO can reach 10E-11. In this way the highest possible 
coherency between master and slave satellite is ensured. 
Synchronising master and slave satellite internal time base to 
GPS timing is practical and cost effective. 
In the case of GPS receiver unlock, the stability of the STALO 
unit will determine the drift due to aging and temperature 
changes. The idea way is to use atomic clock which is also 
disciplined to the GPS. 
3.2.2 pulse-timing synchronization 
Time synchronization includes pulse-timing synchronization 
and absolute time synchronization. Pulse-timing 
synchronization is used to synchronize the SAR data window 
start for the transmit/receive radar and the receive-only radar so 
that both radars can be triggered at a fixed time delay to meet 
the INSAR swath. Pulse-timing synchronization accuracy cr r 
should be less than 0.01 times of the PRT (pulse repetition time) 
to provide sufficient overlap of swath, so <J r < 2.5us is 
required. 
GPS receiver outputs pulse-per-second (PPS) signal once a 
second. The rising edge of PPS which define each second start 
has an accuracy of 50ns. PPS signal is suitable for appropriate 
positioning of the data window and can be used for timing pulse 
transmission and range estimation for Distributed SAR Satellite. 
The receiver of slave SAR is always triggered at a fixed time 
delay after the signal is transmitted to compensate for the range 
difference between the distributed satellites. 
3.2.3 bsolute Time Synchronization 
Absolute time synchronization accuracy cr T is also essential 
for DSS. Absolute time synchronisation needs to be used to 
trigger the data acquisition events and precisely timestamp the 
SAR acquisition data and baseline estimation data. cr T should