IGS LEO fundamental differences with land-based IGS stations

• Differences in tracking geometry
• Differences in signal propagation, in particular atmospheric delays
• Differences in the data flow between receiver and IGS Analysis Center
• Differences in the way in which the data is processed

(A) Differences in tracking geometry
 

Land-based GPS tracking receiver		GPS receiver on-board a Low Earth Orbiting satellite
The Earth-fixed coordinates of the antenna phase centre show variations due to the following effects: Tidal effects, including permanent tide and ocean loading Tectonic plate motions Incidental shifts due to station maintenance, antenna replacement, Earth quakes. Dynamic estimation of the station coordinates (different centers apply different estimated values)	1	The Earth-fixed coordinates of the antenna phase centre show variations due to the following effects: Daily Earth rotation Precession and nutation of the Earth's rotation axis Polar motion All variations of the inertial coordinates mentioned below
The inertial coordinates of the antenna phase centre show variations due to the following effects: Daily Earth rotation Precession and nutation of the Earth's rotation axis Polar motion All variations of the Earth-fixed coordinates mentioned above	2	The inertial coordinates of the antenna phase centre show variations due to the following effects: Orbital motion of the LEO spacecraft Attitude motion of the LEO spacecraft
The antenna coordinates are in general determined on the basis of the GPS tracking data itself. Any offsets between the IGS reference frame and other geodetic reference frames will not be observable through the data collected by such a GPS receiver.	3	The antenna position is determined via known physical offsets with respect to the centre of mass of the spacecraft, while the latter follows from the precise orbit determination process of the LEO. Depending on the presence of non-GPS tracking systems and their relative importance with respect to the GPS data, two cases can be considered: GPS is the dominant tracking data type for the LEO, so that the LEO orbit is implicitly determined with respect to the constellation of GPS satellites, i.e. the IGS reference frame.  GPS is not the dominant tracking data type, but the LEO orbit is essentially determined by some other tracking system (e.g. DORIS, SLR).  In the second case, any offsets between the IGS reference frame and another geodetic reference frame (e.g. the network of DORIS stations) will be observable through the data collected by such a LEO GPS receiver.
The inertial velocity of the tracking receiver is smaller than that of the tracked GPS satellite. Pass lengths are mainly determined by the motion of the GPS satellite, and are relatively long.	4	The inertial velocity of the tracking receiver is larger than that of the tracked GPS satellite. Pass lengths are mainly determined by the motion of the tracking receiver, and are relatively short.
The tracked number of GPS satellites is limited by  The number of receiver channels that can track simultaneously. For modern land-based receivers this is usually no issue, because more GPS satellites can be tracked (12) than can ever be visible at the same time. The cut-off elevation that is applied in the data processing. Typical values are 10 degrees or 15 degrees. Obstacles on the local horizon, such as trees, building, mountains  Changes in the subset of tracked GPS satellites are relatively infrequent because they are typically due to rising and setting GPS satellites.	5	The tracked number of GPS satellites is limited by  The number of receiver channels that can track simultaneously. Early LEO GPS satellites had (software) limits of 8 GPS simultaneous satellites. The cut-off elevation that is applied in the data processing. Typical values are 5 degrees or 10 degrees. Obstacles on the local horizon, such as solar panels, thruster assemblies, instrument booms  Attitude variations of the spacecraft Changes in the subset of tracked GPS satellites are relatively frequent because they are typically due to the orbit and attitude motions of the LEO spacecraft itself.
The global coverage of land-based IGS tracking data is conditioned by geographical and / or political considerations. Global coverage is relatively inhomogenous, although the number of receivers is large.	6	The global coverage of IGS LEO data is conditioned by the LEO orbit. Global coverage is homogenous,   even if the number of receivers is small. The LEO will typically perform 10 to 14 orbital revolutions per day, so that every orbital revolution of a GPS satellite can be tracked during 5 to 7 passes over the same LEO satellite (depending on LEO inclination).
Baselines between land-based stations are limited by the diameter of the Earth.	7	Baslines between LEO satellites or between a LEO receiver and a land-based receiver are limited by the semi-major axis of the LEO orbit.
Estimation of the receiver coordinates simultaneously with the GPS orbits will hardly affect the estimation of Earth rotation parameters, because a network of fiducial sites must always be kept fixed, implicitly defining the reference frame.	8	Estimation of the LEO orbit simultaneously with the GPS orbits may improve the estimation of Earth rotation parameters, because the orbital properties of the LEO (inclination, secondary tracking systems) may provide observability that the GPS orbits do not offer.

Comments

Ref. 1 and 2:

• The effects of Earth rotation appear for the land-based GPS receiver in the inertial coordinates, but for the LEO receiver in the Earth-fixed coordinates. Orbit determination for the GPS constellation is typically done in the inertial frame. If at some point in the distant future a LEO-only tracking network could be realized, effects of Earth-rotation (notably, errors in polar motion) would no longer affect the GPS orbits. The Earth rotation parameters could still be determined from land-based GPS tracking of course, and possibly with greater precision. The IGS LEO Pilot Project will not be in a position to analyse this possibility, other than through simulation, because there will not be more than 4 to 5 LEO satellites available during the course of the Pilot Project.

• The first LEO satellite that can be used for analysing possible reference frame deficiencies in the IGS with respect to DORIS or SLR is JASON-1 - see the reference frame collocation analysis.

• The faster changing tracking geometry has the advantage of better temporal decorrelation of consecutive observations, but brings the difficulty of a larger number of cycle slips.

(B) Signal propagation
 

Land-based GPS tracking receiver		GPS receiver on-board a Low Earth Orbiting satellite
The signal between a GPS satellite and a land-based receiver is in general corrected for the following travel path delays: Satellite antenna offset (from GPS spacecraft centre of mass) GPS satellite clock bias Travel time of the signal (which alters the transmission time tag for a known receiver time tag) Effects of general relativity Ionospheric delay Tropospheric delay Station clock bias Possible station antenna offset with resepct to (monument) station coordinates Some of these effects will be different for L1 and L2 frequencies, and some effects may be eliminated by constructing alternative observables (e.g. single-, double-, triple differences, iono-free combinations).	1	The signal between a GPS satellite and a LEO receiver is in general corrected for the following travel path delays: Satellite antenna offset (from GPS spacecraft centre of mass) GPS satellite clock bias Travel time of the signal (which alters the transmission time tag for a known receiver time tag) Effects of general relativity Ionospheric delay LEO satellite clock bias Satellite antenna offset (to LEO spacecraft centre of mass) The main difference with respect to land-based data is the absence of a troposphere delay and the different computation of the receiver antenna offset. Above a certain elevation and/or LEO height the ionosphere delay may also be neglected. Alternatively, GPS satellites that rise or set at the Earth's horizon will allow for occultation measurements that can be of interest for IGS ionosphere or troposphere products.
The region of ionosphere and troposphere that affects the station's observations is constant, and forms the visibility mask for that station (for the GPS satellite constellation). Some stations will be more affected by atmospheric delays than others, depending on their geographic location.	2	The global coverage of the LEO orbit means that the same GPS receiver will be affected by different regions of the ionosphere within very short period of time.

Comments

Ref. 1:

• The precision with which troposphere delays can be accounted for in land-based GPS data is excellent, and the absence of troposphere delays may hardly compensate for the less accurately known antenna position of the LEO.
• Occultation measurements offer such a wide area of investigation that the IGS LEO Pilot Project will limit itself to maintaining informed on developments in this field, rather than trying to undertake such research within the framework of the Pilot project.

• The advantage that a LEO can offer in this respect is a better decorrelation between daily cycles in the ionosphere activity and daily cycles in the station coordinates, Earth rotation, or GPS orbits.

(C) Data flow
 

Land-based GPS tracking receiver		GPS receiver on-board a Low earth Orbiting satellite
The operation of land-based IGS receivers is relatively straightforward, so that many different organisations operate (part of) the IGS tracking network. The IGS does not depend on any of these agencies in particular, and there is sufficient redundancy in the network to ensure that temporary or permanent loss of a sub-network will not affect the quality or availability of IGS products.	1	Only a limited amount of LEO satellites will ever be available to IGS operations at any moment in the foreseeable future. The operating agencies decide at will if and when their LEO GPS data is made available to IGS processing, and at what latency. Working relations between IGS and the typical LEO agencies are excellent (NASA, ESA, GFZ, CNES, etc.). However, if IGS products become dependent in any way on the availability of specific LEO data, the monopoly position of the involved LEO operator might compromise the independence of IGS.
The received GPS data is immediately available for transfer e.g. to a regional IGS data center. Real-time GPS data transfer is being developed for various applications, in which the latency between the moment of observation and the moment that the measurement arrives at a central processing facility is less than one second.	2	The LEO GPS data is accumulated on-board until the moment that a suitable telemetry downlink is established to a master station on Earth. For most LEO missions such downlinks take place once every orbit, while LEO orbital periods are 1.5 to 2 hours. The latency between the moment of observation and the moment that a measurement arrives at a central processing facility will typically be equally long.
The quality and properties of land-based tracking data is homogenous, because the IGS has defined clear standards that must be complied with by the IGS stations, and these standards are maintained by systematic monitoring of the station's performance.	3	The quality and properties of LEO flight receiver data varies notably from one LEO mission to the other, so that it is difficult to set standards and to maintain them in some systematic way.

Comments

Ref. 1:

• This is a political issue, but a relevant one. Restricted availability of current LEO data is certainly affecting the rate of progress in the IGS LEO Pilot Project. On the other hand, it could be argued that the relative monopoly position of LEO operators is comparable to that of the six IGS Analysis Centres, most of which are in fact controlled by the same few agencies that could operate a LEO mission (NASA, ESA, GFZ, ...).

• 'Quality and properties' does not just refer to SNR, clock stability and other technical matters related to the LEO flight receiver. Equally important are aspects like: quality of the LEO orbit determination, POD method used for the LEO (smooth dynamic orbit, noisy kinematic orbits), or possible availability of LEO combination solutions for orbit or clock. See also 'data processing' below.

Comments

Ref. 1:

• Analysis Centers like GFZ, JPL, ESOC have state of the art LEO groups within their organisation, but LEO experts and processing software are not normally identical to those of the IGS Analysis Centre.