GB2368738A

GB2368738A - System for space vehicle range-rate and integrated range-rate measurements

Info

Publication number: GB2368738A
Application number: GB0016130A
Authority: GB
Inventors: Robert James Ely
Original assignee: Individual
Current assignee: Individual
Priority date: 2000-07-03
Filing date: 2000-07-03
Publication date: 2002-05-08
Also published as: GB0016130D0

Abstract

A system for performing range-rate and ingetgrated range-rate measurements between two transmit-receive units such as a ground station and a space vehicle in which it is not necessary to generate a coherent turnaround ratio between the receiver and transmitter frequencies at the remote unit. The system effectively makes Doppler measurements at the remote unit as well as at the local unit, transmits some digitally encoded data from the remote unit to the local unit, and enables the range-rate as well as the remote reference frequency to be calculated.

Description

System for space vehicle range-rate and integrated range-rate measurements.

This invention relates to a method of determining the radial velocity of a space vehicle relative to a ground station or another space vehicle. Consecutive rangerate measurements can be added in a non-destructive manner to provide a highresolution determination of the range with an unknown integration constant.

BACKGROUND Range-rate and integrated range-rate measurements for space vehicle navigation are well known. Traditional methods involve a coherent turnaround ratio between the carrier frequency received by the receiver on-board the space vehicle and the carrier frequency of the signal transmitted from the space vehicle. Disadvantages of the traditional system are: 1) The space vehicle on-board transponder is complicated by the need to generate the coherent turnaround of frequency.

2) The space vehicle transponder normally has to operate in non-coherent mode for some of the time (e. g. when there is no uplink signal but it is still necessary to transmit telemetry data from the space vehicle to the ground station. When the transponder switches between non-coherent mode and coherent mode there is normally a loss of telemetry at the ground station due to the step change in frequency from the space vehicle transponder.

3) The transmit frequency from the space vehicle is not fixed so that any change of group delay with frequency in the space vehicle transmitter will affect accuracy.

4) The fixed turnaround ratio required between the receiver and transmitter frequencies restricts the possibilities for frequency allocation within the limited capacity frequency bands allocated to space communication.

5) The Doppler frequency offset seen at the ground station is typically twice as large when coherent frequency turnaround is used, increasing acquisition time and requiring wider tracking loop bandwidth in the ground station receiver.

OBJECT The object of this invention is to enable range-rate measurements to be made using a non-coherent transponder on the space vehicle. This will result in simplifying the design of the transponder, eliminating the problems of switching between non-coherent mode and coherent mode, allowing freedom of choice in allocating transmit and receive frequencies (which may be in different frequency bands), and reducing the Doppler offset seen at the ground station during rangerate measurements.

The principle used is to make measurements of the received frequencies at both ends of the communicaions link, in each case using a local frequency reference. Data obtained from the measurement on-board the space vehicle is then telemetered to the groundstation to enable the two measurements to be combined to calculate both the frequency of the on-board reference oscillator and the integrated Doppler shift over a measurement interval, thus providing a measurement of the range-rate.

This invention provides for a space-vehicle transponder in which the transmitted frequency is fixed by being derived from an on-board master oscillator. The space vehicle receiver is able to measure the total phase change of the received signal over a period of time using the same master oscillator as a reference. The equipment at the ground station consists of a transmitter which operates at a frequency derived from a ground based master oscillator, and a receiver which is able to measure the total phase change of the signal received from the space vehicle over a period of time using the same ground based master oscillator. The results of the phase-change measurements made by the space vehicle transponder must be relayed to the ground station via a telemetry channel. The system can be used to determine accurately the frequency of the master oscillator of the space vehicle as well as the range-rate.

A preferred embodiment of the invention will now be described with reference to the accompanying drawings in which: FIGURE 1 shows a typical application in which the rate of change of the range between the ground station and the satellite, v, is to be measured by determining the Doppler shift of the signal frequencies.

FIGURE 2 shows a block diagram of part of a typical space-vehicle transponder that would enable the measurements to be made.

DESCRIPTION A typical application for this invention is to measure the range-rate between a ground station and an orbiting satellite. The invention is equally applicable to making range-rate measurements between two space vehicles. The communications links used for the range-rate measurements will normally, but not necessarily, be the same links used for spacecraft telecommand, telemetry, and ranging operations. This is illustrated in FIGURE 1 in which v is the rate of change of the range between the ground station and the satellite, FT is the frequency of the signal transmitted to the satellite from the ground station, Fr is the frequency of the signal received by the satellite, Fro is the receiver centre frequency, Fo is the frequency of an on-board reference oscillator, Fd represents the difference between actual received frequency from the receiver centre frequency, the average value of Fd/Fo over a measurement interval is transmitted to the ground via a communications link, Ft is the frequency of the signal transmitted from the satellite, and FR is the frequency as received at the groundstation.

When a signal is transmitted from the ground station with a frequency FT the signal will be received at the space vehicle with a frequency Fr which is different from FT due to the component of the space vehicle velocity in the direction of the ground station due to the Doppler effect. Similarly, when a signal is transmitted from the space vehicle with a frequency Ft it will be received at the ground station with a

frequency FR. The relationship will be :

Fr FR H FT"F u i T/where c is the speed of light.

V /c Fyc

and therefore :

FR Fr writing-,-= q we can obtain : FT Ft l-q v = c l+q

In a traditional range-rate system the ratio Ft/Fr is a constant when the space vehicle transponder is operating in coherent mode, so that it is only necessary to measure the ratio FR/FT at the groundstation in order to calculate v. However, in this invention, the value of Ft/Fo will be fixed predetermined value, where Fo is the frequency of a stable reference oscillator on-board the space vehicle. In general the accuracy of the on-board reference frequency will not be as good as the ground based frequency reference and will be subject to variations due to temperature and ageing.

The receiver on board the space vehicle will measure the received frequency relative to Fo and so obtain the ration Fr/Fo. Sufficient information will then be transmitted from the space vehicle to the ground station such that the value of Fr/Fo is known at the ground station. This will enable Fr/Ft to be calculated and thus the range-rate, v, to be calculated:

Where Ft/Fo is a known constant, Fr/Fo is obtained from telemetered data.

It would be possible to simple transmit the value of Fr/Fo, but to reduce the quantity of data to be transmitted it may be preferable to subtract the nominal constant value Fro/Fo (which will be predetermined and known at both ends of the communication link) and only transmit the difference, Fd/Fo = (Fr/Fo) - (Fro/Fo).

The telemetered value will generally arrive at the groundstation some time after the on-board measurement has been made due to delays in the telemetry system. To obtain the best possible accuracy from the system any delays introduces in the telemetry should either be predictable or calculable form time-tag data added to the measurement. This may not be important for configurations where the range-rate, v, varies only slowly with time.

Having calculated v, the change in range can be calculated from :

t R-Ro = f v. dt to

where Ro is the initial range.

The frequency of the on-board reference oscillator can be calculated from :

(Fa) 1+7c (Fa) Fo =1 /as the ratio F) is predetermined and known.

-Z Ft 1-Y Ft Fc A POSSIBLE IMPLEMENTATION FIGURE 2 shows a block diagram of part of a typical space vehicle transponder.

The received signal at frequency Fr is first down-converted (1), either by a superheterodyne process or by sub-harmonic sampling, and finally converted to baseband quadrature signals by a variable frequency element (2). In a digital implementation the variable frequency element will normally be a number controlled oscillator (NCO) (3). The NCO is clocked at a constant frequency, say b * Fo, and on every clock pulse a digital word, w, called the phase increment word, is added to an accumulator (any overflow being ignored). The output of the phase accumulator, 0, represents the phase of the variable frequency element. The phase increment word, w, is updated at fixed time intervals, m/Fo, where m is a known constant. The value of w is derived from the baseband signals in such a way as to keep the receiver phase-locked to the received signal, this function being performed by the digital carrier tracking loop (4). Each time the phase increment word, w, is updated the value of w can be added in an accumulator (6) which generates the sum, W, of all phase increments over a pre-determined number, n, of update periods. The value of W is included in the space vehicle telemetry data.

The frequency Ft transmitted from the space vehicle is generated by multiplying the reference frequency by a known constant, k. Note that all frequencies used in the receiver and transmitter are derived from a single reference oscillator (5).

In the frequency plan shown in FIGURE 2 the following equation applies when the

receiver is phase-locked to the received signal.

w. b. P Fr f--Fo ±where d is the number of bits in the NCO accumulator. m n In a measurement period t=-seconds the total phase change of the Fa

received signal is :

W m b P = a m n + 2d cycles, Where n W. m. b 2d cycles, Where n W = L Wi 1=1

is the sum of NCO phase increment words, w, applied during the measurement period. The average value of Fr over the measurement period is :

Fr : = h. F h h = P = a+ W. b t were t n. 2d

So to determine the value of h at the ground station it is only necessary to telemeter the value of W as the other parameters in the equation are determined by the design of the transponder and hence known. The telemetry system should provide a means of relating the time period from which the value of W was obtained to the time that the value appears in the telemetry stream.

Suppose, for example, that range-rate measurements are to be made once per second. The digital carrier tracking loop may be updated at a rate of, say, 100 kHz, so that n = 100000. If the NCO word, w, is 32 bits then the value W will not be greater then 47 bits. As W only has to be transmitted once per second the telemetry data rate required to support the range-rate function is not more than 47 bits per second which will generally be small compared to available telemetry rates.

(in practice it is only necessary to accumulate the offset from the receiver rest frequency rather than the complete NCO phase increment word so that the number of bits required will be reduced).

As a further refinement the accumulator (6), instead of being reset at the start of each measurement period, could add the n values of w obtained in a measurement period to the previous sum. Assuming that the values are represented in twos complement form, the value of W can be recovered at the ground station by taking the difference between consecutive transmitted values. This method would be of benefit when there is a risk of loss of data in the telemetry stream due to bit errors when the signal to noise ratio is poor because when data is missing it is still possible to find the total phase change between the values either side of the outage. The effect of missing data is simply that the measurement interval is increased.

The system used at the ground station for generating FT and measuring FR can use the same architecture as the system described above for generating Ft and measuring Fr.

Other embodiments of the invention will be apparant to those knowledgeable in the field. For example frequency counters could be used to perform the measurements instead of accumulating NCO control words and feed-forward carrier correction could be used insead of feedback. The signals used for the communications links may be modulated with telecommand, telemetry, ranging, or other data provided that the modulation schemes used allows for recovery of the carrier frequency, and preferably phase, in the receivers.

Claims

CLAIMS 1. A range-rate, and integrated range-rate, measurement system using electromagnetic signals between two transmit-receive units, the system comprising : at the remote unit a stable oscillator used as a local frequency reference, a signal transmitter operating at a frequency which is the local reference frequency multiplied by a known constant, a receiver with the capability to measure the average frequency of the received signal over a certain time period (or, equivalently, capable of measuring the total phase change of the received signal over the time period) relative to the local reference frequency, and a means to digitally encode and transmit the measurement data to the measurement unit; at the unit performing the range-rate measurements, a transmitter transmitting a signal to the remote unit, a receiver receiving the signal transmitted by the remote unit, a means to measure the ratio of the received frequency and the transmitted frequency, a means to receive telemetered data from the remote unit Including digitally encoded information that will enable the ratio of receive to transmit frequencies at the remote unit to be calculated, a means to provide the local and remote measurements or the results of calculations using the local and remote measurements as an output of the system in order to provide range-rate and/or integrated range rate information to a user of the system.
2. A range-rate measurement system as in Claim 1 which Includes a means to calculate the frequency of the reference oscillator at the remote unit relative to a reference oscillator In the local unit.
3. A range-rate measurement system as in Claim 1 in which either or both receivers use an NCO to generate the receive frequency and the phase Increment words loaded into the NCO over a period of time are summed in order to produce the total phase change (or average frequency) measurement.
4. A range-rate system as in Claim 1 wherein the communication links may be modulated with information or other data such as telecommand, telemetry, and ranging signals, and where any modulation scheme may be used.
5. A range-rate system as In Claim 1 wherein the measuring unit IS at a ground station and the remote unit is in an orbiting space vehicle.
6. A range-rate system as in Claim 1 wherein the measuring unit is in an orbiting space vehicle or space station and the remote unit is in an orbiting space vehicle.
7. A range-rate system as in Claim 1 wherein the measuring unit is In a planetary orbiter and the remote unit is in a planetary lander or a free flying sub-satellite or probe.