Tracking

General

TDRSS tracking support provides all position, time, and frequency data necessary to maintain precise customer spacecraft orbit prediction (acquisition), orbit determination (tracking), and attitude determination and control.

This portion of the knowledge base includes the following additional TDRSS tracking information:

Overview of the TDRSS Acquisition/Tracking Support Cycle
Nominal Tracking
Cross - Support Tracking
Tracking Accuracy
Attitude and Control Support
Tracking of the TDRSs
Tracking Data Computed at the Customer Flight Platform (TONS)

Overview of the TDRSS Acquisition/Tracking Support Cycle:

The TDRSS acquisition/tracking support cycle may be summarized as the following:

The GSFC Flight Dynamics Facility (FDF) can provide customers with orbit planning and accuracy studies during the mission planning phase.
During customer S/C operations, the White Sands Complex(WSC) extracts tracking data (range and range rate data ) from the customer's downlinked telemetry link to TDRSS.
WSC sends, via NISN, tracking data in TDRSS Tracking Data Format to GSFC's Flight Dynamics Facility (FDF)
The FDF places this data into temporary storage and periodically makes ephemeris (the set of position and velocity vectors ) out of the buffered data. After computing an orbit, the FDF uses this information to generate a time history of where the satellite was (definitive orbit ) and where it will be (predicted orbit), and the associated velocity at each time point. Short term ephemeris is produced for the next week; long ephemeris is produced for the next month.
Ephemerides are then transformed into acquisition data in an Improved Inter-Range Vector (IIRV) format. IIRVs contain satellite position and velocity, with a reference epoch time.
Once an orbit is computed, it is also quality checked against a previously determined orbit to ensure continuity and accuracy.
IIRVs (commonly called state vectors) are sent to WSC for both the TDRSs and their customers. Updates to these vectors are normally provided several times a day .
WSC uses IIRVs to track the TDR spacecraft and for TDRS antenna pointing angles necessary to acquire and track the customer spacecraft.
Differences between the predicted coordinates and the actual coordinates measured during the support are returned to the FDF as UTDF tracking messages to update the tracking data base. New updated IIRVs can then be produced and the cycle begins again.

The TDRSS Tracking Data Format

The TDRSS tracking data format, a subset of the NASA Universal Tracking Data Format (UTDF), consists of 75 bytes of contiguous information comprising one sample of data. Tracking data messages (TDMs) consist of one or more standard NASA/TDRSS 4800 bit data blocks with each block containing from 1 to 7 tracking samples. The number of samples is a function of simultaneous TDRSS service support.

Nominal Tracking Support

TDRSS tracking support can be accessed through any of three types of forward and return link TDRSS services (MA, SSA, KSA), depending on tracking requirements. The type of tracking services available depends on the Data Group and Mode used for support.

Although the minimum requirement for TDRSS tracking support is a single return link (for one way Doppler support), typical scenarios for TDRSS tracking support include an active forward and return link to the customer's flight platform using either the MA, SSA, or KSA services.

Types of TDRSS Nominal tracking support include:

MA, KSA and SSA one-way Doppler measurements. The customer S/C one-way Doppler measurements is available for non-coherent DG2 and DG1 mode 2 operations.
MA two-way range and Doppler measurements. The customer S/C two way Range and Doppler measurements is available for coherent DG1 mode 1 operations.
(K and S) SA two-way range and Doppler measurements. The customer S/C two way Range and Doppler measurements is available for coherent DG1 mode 1 and mode 3 operations.

Various sampling rates of Range and Doppler data can be provided for real-time delivery to the GSFC FDF (e.g., 1/sec., 1/5 sec.)

Customer Requirements for Nominal Tracking Support

Two Way Ranging For nominal tracking services, the two-way ranging can only be done when the customer service is coherent, at least one of the return data channels is PN spread, and the return link PN code is synchronized with the PN code of the TDRSS forward link received by the customer S/C.

Two-way Doppler The two way Doppler (Range Rate) measurement requires a return link carrier that is coherent with the forward link carrier. "Coherent" means the return link is coherently related in frequency (as a function of the customer S/C transponder) to the TDRSS forward link received by the customer S/C.

One-way Doppler The one way Doppler measurement only requires an active return link carrier.

Cross-support Tracking Services

In addition to these nominal services, S-band customers can receive simultaneous cross-support tracking services which provide two-way Range and Doppler measurements to customers using either MA or SSA on the forward link and to the opposite type of S-band service (MA or SSA) on the return link (e.g., the MA on the forward link crossed with SSA on the return link).

TDRSS Tracking Accuracy

TDRSS tracking support, along with orbit determination provided by the FDF, is capable of providing 50-100 meter accuracy (week to week) with TDRSS supporting customer S/C signal Doppler frequencies up to + or - 230 kHz and Doppler rates up to 1.5 kHz/sec.

Attitude Control Support

Monitoring and correction of S/C attitude (altering of Roll, Pitch, and Yaw orientation) is the primary responsibility of the customer, with FDF assisting.

Parameters that describe attitude orientation are downlinked within housekeeping telemetry to customer POCCs which can strip out this data, reformat it and send it to the FDF. The FDF can provide customers with both real-time and near-real-time attitude control information based on mission requirements, and provide this information in the form of commands to the POCC for making attitude adjustments.

Tracking of the TDRSs

The TDRSs themselves are tracked by the DSMC scheduling tracking services to ground-based transponders known as the Bilateration Ranging Transponder System (BRTS). Each TDRS is scheduled twice every four hours to track the BRTS transponders. Since the BRTS locations are precisely known, the FDF can accurately compute the TDRS locations. TDRS locations are ,in turn, provided to the customer for their tracking of the TDRSs.

Tracking Data Computed at the Customer Flight Platform (TONS)

GSFC has also developed a navigation tool called the TDRSS On-board Navigation System (TONS) which allows basic tracking measurement data to be computed at the customer flight platform, instead of on the ground at FDF. TONS produces both raw radiometric data and orbit determination data on-board through the combination of a second generation TDRS transponder, and external ultra stable oscillator (USO) a Doppler extraction card, and on-board navigation and signal processing software. TONS provides tracking measurement and orbit determination with little additional cost, weight, and volume impact on the customer's flight platform.

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