Re: Does ASTER use GPS?
- From: matt weber <mattheww50@xxxxxxx>
- Date: Thu, 29 Dec 2005 16:09:25 -0700
TDRSS enjoys one very very important advantage over most users. It is
located in space, consequently the tiny variations in permittivity and
permeability that plague ground users, and limits accuracy even with
WAAS to about 3 meters, don't exist for Space based systems. TDRSS
doesn't have to deal with the largest single cause of GPS errors.
On Thu, 29 Dec 2005 21:00:49 +0000, Marc Brett
<marc@xxxxxxxxxxxxxxxxxxx> wrote:
>On 29 Dec 2005 10:01:06 -0800, heleneliz@xxxxxxxxx wrote:
>
>>Hello,
>>
>>I am working on an essay about the TERRA satellite's Advanced
>>Spaceborne Thermal Emission and Reflection Radiometer (ASTER) unit.
>>
>>So far, I've had no success in determining if ASTER makes use of Global
>>Satellite Positionning (GPS) or INS for that matter; would anyone be
>>able to help me with some related information?
>>
>>ASTER provides stereoscopic images and digital elevation data, using
>>ground points which are called GCPs. Which positioning system is used
>>by ASTER to make sure it is synched with those ground points?
>
>http://www.gds.aster.ersdac.or.jp/gds_www2002/entrance_e/gdshistory_e/gdshistory_e.html
>
>Among the subsystems that are performing significantly better than spec
>requirements, is the TDRSS On-Board Navigation (TONS) System. Terra is flying an
>autonomous onboard navigation system to provide accurate orbital parameters to
>the spacecraft in real time. This system represents the first time NASA has
>flown an onboard navigation as the operational system for orbit solutions.
>Scientists also receive the TONS output in their data stream.
>
>TONS measures the Doppler data off the forward command signal and processes it
>in onboard software with a sequential estimation algorithm to produce the real
>time outputs. The accuracy requirement for TONS is 150 meters in position and
>0.16 meters per second in crosstrack velocity, 3 sigma.
>
>TONS performance has far exceeded the requirement. TONS performance at this time
>is on the order of 7 meters, one sigma in position and better than 0.015 meters
>per second in crosstrack velocity. After on-orbit tuning, TONS is expected to
>provide Terra with onboard position knowledge to better than 5 meters, one sigma
>and 20 meters, 3 sigma. TONS performance has been monitored during the Terra
>ascent maneuvers and indicates excellent recovery within the first few
>measurement updates after the maneuver. TONS filter reconvergence occurs at the
>beginning of the second post-maneuver contact. TONS also estimates the local
>oscillator frequency, a drag correction factor, and a TDRS measurement bias for
>Terra.
>
>All on-orbit requirements have been exceeded and navigation operations have been
>performed nominally. TONS data was also used to calibrate the spacecraft clock
>to aid the operational clock correlation system. The Goddard and Lockheed
>Navigation Team have done an extraordinary job in developing this navigation
>subsystem. Congratulations to everyone on the Nav Team. This performance should
>allow the scientists to geolocate the images even better than expected.
>
>On a few occasions this past week, problems resulting in solid state recorder
>buffer overflow, or situations coming close to overflow have occurred. A Team
>has been formed to characterize the magnitude of the problem, determine the root
>causes, develop metrics to track performance, and evaluate improvements in the
>process.
>
.
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