Science Measurement Objectives
The following table presents a concise summary of exactly what measurements GRACE will obtain and the instruments that are used to obtain them, as well as how many spacecraft are involved in each measurement.
Science Application
 Measurement
 Instrument
 # of spacecraft needed for measurements

Earth Gravity Field 
InterSatellite Range Change Link 
KBand Microwave 
2 
Earth Gravity Field Accelerations 
NonGravitational 
Accelerometer 
2 
Earth Gravity Field 
Satellite Orbits 
Black Jack GPS Receiver 
2 
Atmospheric Occultation 
GPStoGRACE Phase Change 
Black Jack GPS Receiver 
1 
This science data in conjunction with ancillary data, will then be used to gain estimates of spherical harmonic coefficients of the Earth's gravitational potential.These coefficients are necessary to construct the 30day maps of gravitational potential that will be the end product from the primary GRACE measurements.
Gravity Measurement Error
Intersatellite Range Measurement: 
< 4 µm @ twice/rev < 10 µm total @ 0.1 to 0.0001 Hz 
NonGravitational Acceleration Measurement:

< 10 10 m/s2 noise (+1/f) < 4 x 10 12 m/s2 tones. 
Precision Orbit Measurement: 
5 cm absolute < 0.2 mm, relative in plane 
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