A_3-axis Angular Displacement Sensor (ADS) is flown on 
Landsats 4 and 5 fo measure spacecraft motion experienced 
at the foot of the TM instrument, using.outputs of gyros 
to measure distrubances below 2Hz and the ADS to measure 
disturbances between 2Hz and 125Hz. The following jitter 
levels have been observed (note that" yaw and pitch axis 
jitter is negligible): 
  
  
Operating Modes Roll Axis Jitter (Peak to Peak) 
TM only 2 arc second 
TM and High Gain Antenna 3 art second 
TM and MSS 3.5 arc second 
TM, MSS, and High Gain 5.5 arc second 
Antenna 
The above data show that ground systems must correct 
image data for jitter effects in order to achieve 
subpixel geometric accuracy. 
An experimental GPS package consisting of a 
receiver/processor assembly, antenna, preamp and stable 
oscillator was placed on Landsat-4 as the first 
spaceborne navigation set to use the GPS. A GPS package 
is also included on Landsat-5. The objective of the 
experiment to determine the ultimate accuracy with which 
the GPS can provide real-time onboard estimates of 
satellite position. Experiment results to date can be 
summarized as follows. 
During the GPS evaluation period only 4 of the planned 18 
NDS satellites were available. As a result, all Landsat 
revolutions contained periods of up to 40 minutes when no 
NDSs were in view. 
The memory system in the GPS is sensitive to radiation 
induced bit flips.. Software to detect and.correct bit 
flips when they occur is currently being developed. 
Reference ephemeris comparisons indicate that errors in 
Landsat position and velocity from GPS are consistent]y 
less than 50 meters and 6 centimeters per second during 
periods of good NDS visibility. When GPS meaurement gaps 
occur, these orbit errors grow exponentially; but the 
peak position errors during a Landsat revolution are 
generally less than 1500 meters. 
In summary, some additional work is necessary to solve 
for the onboard memory radiation induced bit flips, -to 
optimize the Kalman filter 50 peak orbit errors will be 
reduced, and to refine the operational software. It 
follows that,.upon completion of this: wOrk,. GPS will 
become an attractive alternative for supplying onboard 
ephemeris to future spacecraft systems. 
Direct experience with Landsat-5 provides tne following 
performance measures for critical subsystems: