CA2211340C - Method for automatically lowering the acquisition and tracking thresholds of spread spectrum codes received in orbit - Google Patents

Abstract

The present invention relates to a method of autonomously reducing the acquisition and tracking thresholds of spread spectrum codes received in orbit by a receiver accessing an orbital navigator internal to or external to said receiver, a phase loop and a loop. of code. The code loop, responsible for tracking the pseudo-random codes, is "pushed" by the fine speed help, and makes up for the error between the actual speed and the calculated speed. The code loop, responsible for the acquisition of the pseudo-random codes, is also "pushed" by the aid of the finite speed e, the search for the phase of the received code being carried out around a phase prediction maintained by the fine speed help from the orbital browser.

Description

METHOD OF AUTONOMOUS REDUCTION OF THRESHOLDS
ACQUISITION AND CONTINUATION OF CALCULATION CODES
i SPECTRUM RECEIVED IN ORBIT
DESCRIPTION
Technical field The invention relates to a method of the autonomous reduction of the acquisition and continuation of spread spectrum codes received in orbit.
State of the art The invention combines three elements of base.
- a spectrum signal receiver spread out;
an onboard orbitography filter;
- a threshold reduction technique by precise help in radial speed.
We will consider below each of these three elements.
~ The receiver can be any equipment receiving spread spectrum signals, edge of a satellite (references [1], [2], [5)). These signals can be emitted by other satellites in orbit or from fixed points on the ground. As for example, these receivers may be of the type.
- GPS receiver, GLONASS, GNSS1, GNSS2;
- spread spectrum transponder;
- DORIS NG receiver.

2 Constellations of GPS satellites and, GLONASS are respectively described in the references [3] and (4).
GNSS1 designates the complements geostationaries with GPS and / or GLONASS using the INMARSAT 3 satellite navigation payloads.
GNSS2 designates the future "civil" constellation of navigation satellites.
~ DORIS NG is a system project global radionavigation and radiolocation space based on the use of signals spread spectrum transmitted by ground beacons, and received by satellites in orbit.
~ The orbitography filter is a digital processing, located in the receiver, by example. It uses the raw measurements made by these last, that is to say measures of pseudo-resistance and pseudo-speed related to signal transmitters spread spectrum (ground or orbit). These measures are processed to autonomously determine the orbital parameters and / or the position and speed of the carrier satellite. The definition of these measures is given in reference [5]. The filter can be following types, given as an example.
- Kalman filter (see reference j6));
- simple least squares filter;
- recursive least squares filter.
This filter is also capable of determine the following parameters.

3 Di - Distance between the satellite and .
the director n i.

~ Ti - Time gap between the clock of the receiver and the director's clock n i.

Vi = Di - radial velocity between and the satellite the director n i.

- Relative drive between the clock of the director ni and the clock of the rcepteur.

The filter can therefore estimate the pseudotypes PDi and pseudovitess es PVi.

PDi - Di + C. ~ Ti PVi - Vi + C.Ofi The quantities quoted can be estimated even if the signals from transmitter no.
are not processed by the receiver and the filter of associated navigation, provided that the position, the speed and the clock coefficients of said transmitter can be estimated or known.
The orbital browser estimates these pseudo-resistances and these pseudovitesses with the 10PD and SPV specifications.
The orbital navigator can receive satellite maneuver description remote controls carrier. These maneuvers can be described by OVx (to) parameters; OVy (to) and ~ Vz (t0), where the OVi represent the components of the impulse in speed scheduled on the date to.
The maneuvers are described with a precision noted BVx, BVy, 8Vz for the three axes. The overall accuracy of the description of the maneuver is $ V, with

4 sv = svX + sv ~ + svZ
~ Threshold reduction technique by precise help in radial velocity (or pseudovitesse radial) applies to receivers equipped with a (or more) coupled phase loops) with a (or more) loops) of code. It is assumed that these loops are made in digital technology.
When the signal is received with a ratio C / No less than the conventional hooking threshold in assisted acquisition mode, the carrier loop is open, and the digitally controlled oscillator (OCN) is driven by an external speed prediction (or pseudovitness) radial.
Classic hanging mode Assisted acquisition is illustrated in reference [1].
This speed prediction must be fine and comes from a sensor different from the receiver. By for example, said sensor can be typically a inertial unit.
Such a technique is used classically for tracking GPS signals to low ratio C / No equivalent, received by military GPS receivers (C / A code and P code) mounted on weapon planes equipped with inertial units. This technique is called "code only" because only the pseudo-random code is continued up to thresholds very low.
The subject of the invention is a method of autonomous reduction of the acquisition and continuation of spread spectrum codes received in orbit.

Presentation of the invention The invention relates to a method of autonomous reduction of the acquisition and

Continuation of the spread spectrum codes received in orbit by a receiver accessing a browser orbital internal or external to said receiver, characterized in that the receiver having a phase loop and a code loop, the code loop, loaded with the the pursuit of pseudo-random codes, is "pushed" by the speed help, and catch up with the error between the real pseudovitness and calculated pseudovitness, and in that the code loop, responsible for the acquisition pseudo-random codes, is also "pushed" by the help of fine speed, the search for the phase of received code around a phase prediction maintained by this speed helper.
The method of the invention comprises following steps .
- the receiver receives the necessary help to the classic assisted acquisition, which allow the receiver to hang all signals with a report C / No such that C / No> (C / No) a, (C / No) a being the threshold hooking in classic assisted acquisition mode;
- we reduce the thresholds of the codes pseudo-random up to the value (C / No) avf ~ (C / No) avf being the threshold for snapping pseudo-random codes in acquisition mode helped by a fine help in speed, using fine speed from the browser orbital.
The method may comprise a step during which the receiver starts cold without any help or external or internal message, and hangs all the signals with a C / No tel ratio

6 than . C / No> _ (C / No) na, (C / No) na being the threshold snap in unaided mode.
In the case of a control maneuver satellite orbit, the orbital navigator receives the description of said maneuvers, and updates the help speed provided by the browser. So that snapping pseudo-random codes received with low C / No ratios are always possible in case of maneuver, the following conditions must be respected, first order.
~ BPV + $ V ~ <Bei XC
fi where 8PV is the uncertainty about the prediction on the pseudo-speed provided by the browser, in the absence of laborer; where BFI is the predetection band, C la speed of light and fi the frequency of the signal carrier transmitted by the transmitter n ° i.
When a pseudo-random code is continued with a C / No report such as.
~ C / NO ~ avf <C / No <~ C / No ~~, the updates of the characteristic parameters of the transmitters are communicated to the receiver using remote controls External.
The method of the invention consists of reduce the acquisition and prosecution thresholds spread spectrum codes, by receivers for satellites, equipped with an onboard orbitography filter.
This threshold reduction is performed autonomously by said receivers using the invention. This ' Threshold reduction can be spectacular.

WO 97/2022

7 PCT / FR96 / 01870 Brief description of the drawings - Figure 1 illustrates a block diagram a reduced threshold receiver by fine speed help external ;
FIG. 2 illustrates a block diagram a receiver integrating the device of the invention;
FIG. 3 illustrates a variant of the invention.
Detailed presentation of embodiments Figure 1 illustrates a receiver in spread spectrum with reduced threshold by fine speed aid external, only the architecture of digital processing being represented.
This receiver 10 comprises an RF module 11 connected to an antenna 12 whose output signal is entered on a correlator 13 itself connected, output, to a discriminator 14 of the phase loop followed by a loop filter 15, and a co ~~ uta; .eu: 16. A module OCN (digitally controlled oscillator) carrier 17 transmits a local carrier Fi phase e ~ en quadrature to the discriminator 14, and is connected to a other terminal of the switch 16. The switch 16 receives also a signal from a sensor of external speed 18 (inertial unit for example).
The carrier OCN module sends a signal of carrier OCN speed on a first input of a summator 19 connected in output to an OCN module code 20, and a local code generator 21. This generator 21 is connected on the one hand to the correlator 13 and delivers it the local code in phase, and secondly to a discriminator of the code loop 22 and delivers it the local code in advance and the local code late, a

8 loop filter G (p) 23 being disposed between the output of this discriminator 22 and a second input of, summoner 19.
We thus have a phase loop 33 and a code loop 34.
D ~ ", .. r." ~ A - .., ~ The code loop, responsible for the pursuit pseudo-random codes, is "pushed" by the help of speed. In other words, the OCN oscillator of this loop varies the phase of the local code with a equal speed using the external prediction of speed. The code loop and "thrust" catches up the error between actual speed and speed calculated.
The order of this loop must be sufficient to maintain the OCN oscillator slave, which allows to produce pseudo-resistance measures.
Acqui si ti The code loop, responsible for the acquisition pseudo-random codes, is also "pushed" by fine speed help. The search for the phase of received code is performed around a phase prediction (prepositioning distance) maintained by the help of external speed.
The search area of the code phase "
received is smaller than in the case of an acquisition in classic assisted mode. It is the same for the zone ' searching for the frequency of the received carrier. In Indeed, the principle described works if the uncertainty

9 of the AFD Doppler prediction is lower than the band BpI predetection. ' The 8PV uncertainty on the prediction of pseudovitesse must therefore respect the relations following.
OFD <BFI is: 8PV <B ~ 'I x C
where C is the speed of light and fi the frequency of carrier signal transmitted by the transmitter n ° i.
Thus, areas of uncertainty in Doppler and in distance being lower than in the cases the search for energy can be carried out with a local code sweep speed a lot slower than in these classic cases, for a meme duration of search noted T. The acquisition threshold is thus found reduced. Rehabilitation techniques false clashes must be implemented in case of simultaneous reception of several codes pseudo-random, with disparate C / No's.
The following C / No ratios are defined.
(C / No) na - Snap threshold in assisted mode.
(C / No) a - Acquisition mode snap threshold aided classic (C / No) avf - Threshold for hanging pseudo-codes randomized acquisition mode aided by a fine help with speed The definition of thresholds (C / No) na and (C / No) a is detailed reference [1].
he threshold (C / No) avf is a function of several settings.

(C / No) avf - g (T: BPV, BFI) The invention is characterized by the method, next, implemented in a spectrum receiver spread for satellite, equipped with an orbital navigator.

Step 1 (optional)

10 Ze receiver starts cold (without any help or external or internal message). I1 hangs all signals with a C / No ratio such that.
C / No _> (C / No) na The first hung signals can possibly allow the receiver to.
- receive messages allowing determine the position and / or speed and / or clock coefficients of emitters n ° i;
- to converge the orbital navigator, to with the help of the first measures of pseudo-resistance and pseudovitess performed.
This first step is necessary to fully autonomous space missions.
2nd step The receiver receives the necessary aids to classical assisted acquisition. These helpers are few precise (coarse) and type.
1) Date and time of the receiver clock.

11 2) Positions / speeds (or possibly orbital parameters) of the transmitters.
3) Position / velocity or orbital parameters of the carrier satellite.
These aids may come in part or completely from step 1. In this case they are internal to the receiver (eg positions / speeds transmitters may be transmitted by the said transmitters) the autonomy function is retained.
Where all or part of these aids coarse is communicated to the receiver through remote controls, this one is no longer autonomous.
In the case where step 1 is not realized, these aids are obligatorily external to the receiver.
These coarse aids allow the receiver to hang all signals with a report C / No such as.
C / No> (C / No) a The number of pseudo-speed measurements and pseudorange then increases, compared to the case of Step 1, because (C / No) a <(C / No) na ~
The number of these measurements is assumed enough to converge the orbital browser providing the orbital parameters of the satellite carrier with better accuracy than in step 1.

12 Step 3 Step 2 being performed, it is assumed that the accuracy of browser output settings Q
orbital and the characteristic parameters of transmitters is consistent with the smoothness of the aid of speed required to further reduce thresholds snapping pseudo-random codes, up to the value (C / No) Unlike the state of the prior art, fine speed help comes from the orbital browser integrated into the receiver. This help is therefore internal and autonomy is preserved, as represented on the figure 2.
The accuracy of the browser can be further improved. Moreover, in case of degradation the progress of the liaison report with the issuers, the degradation of this accuracy may be limited.
So we have .
(C / No) avf << C / No) a Step 4 In the case of a control maneuver satellite orbit, the orbital navigator receives the description of said maneuvers, and updates the help speed provided by the browser.
For snapping codes -pseudorandom received with low C / No reports always possible in case of maneuvers, the following condition must be met at the first order

13 (sPV + sv) <~ = xc where sPV is the prediction on pseudoviteness provided by the browser, in the absence of maneuver.
Step 5 When a pseudo-random code is continued with a C / No report such as.
(C / No) with <C / No <(C / No) a the demodulation of the data of the message transmitted by the transmitters is not possible.
Indeed, this demodulation must be performed by the carrier loop (phase loop).
This loop is open when the C / No respects the double inequality cited above.
Parameter updates characteristics of the transmitters (positions and / or speeds and / or clock coefficients) must therefore must be communicated to the receiver by means of external remote controls. For example, in the case of a GPS or GLONASS receiver, these parameters may to be the ephemeris of the constellation used.
Figure 2 illustrates a receiver 29 of _ 30 satellite-based spectrum signals which includes an RF module 30, receiving a signal from a antenna 31, connected to a first input of a correlator 32 followed by a phase loop 33 connected to a code loop 34 which also receives the signal of

14 output of the RF module 30, and whose output is connected to a second entry of the correlator 32, and a browser, integrated orbital 35 which receives from the phase loop the data and pseudo-rates if (C / No)> (C / No) a and who sends him a help of fine speed, and who receives from the code loop the pseudoranges.
The integrated orbital navigator receives the description of satellite maneuvers and data External.
~ Figure 3 illustrates a variant of invention, where the orbital navigator is integrated into an onboard computer 36 present in the satellite.
Application examples Applications of the process of the invention concerning the reception of spread spectrum signals at edge of satellites relate to the adverse cases of balance sheets between the transmitters used and said satellites.
For example, these applications can be.
from the point of view of the types of receivers * Navigation using a receiver of constellation of satellites (like GPS, GLONASS).
* Navigation using a transponder spread spectrum. The link budget can be unfavorable at the beginning and end of a station overflight remote control / telemetry (TM / TC). ' * Navigation using a receiver of signals in transmitted spectrum transmitted by a park of ground beacons with type diagram antenna hemispherical. The power transmitted by these tags is supposed to be optimized for use by satellites in low orbit. The link budget is r so supposedly less favorable for a satellite in geostationary orbit, for example.
5 * In-orbit reception of spectrum signals spread out, in a scrambled environment by transmitters unwanted radio. The C / NO ratio equivalent of the received signals is decreased in relation to case of a non-scrambled environment. The balance sheet of the bond is thus degraded and the process of the invention may be necessary in this case.
from the point of view of the orbits * Navigation using a GPS receiver, or DORIS NG in geostationary transfer orbit. A
geostationary transfer orbit can be types following.
- geostationary transfer orbit classical (OTG);
- supersynchronous orbit (OSPS);
- sub-synchronous orbit (OSBS);
- orbit of drift (ODD);
These navigations can be done with two antennas with low gain, if the threshold of attachment signals are weak (see reference [7]).
3o * Navigation using a receiver in - high apex orbit that can be of the following types cited as an example (see reference [8]).
~ - Molniya orbit;
- Tundra orbit;
- Archimedes orbit.

* Navigation using a receiver in circular orbit for satellite constellation of navigation, of the order of twelve hours. Za reception of beacons on the ground with hemispherical antenna is adapted to this case.
* Navigation using a receiver in low orbit, connected to one or more antennas of reception at low cost, and therefore not optimized, but good enough to allow the receiver to perform step 2 described in the invention.

REFERENCES
there [1] "Orbital Navigation With GPS Receiver On The HETE
Spacecraft "(ION GPS January 94, pages 645-656) [2] "ESA Dual-Standard 5-Band Transponder." Versatile TT & C Equipment For Communications Via A Data Relay Gold Satellite Directly With The Ground Network "of JL Gerner (42nd Congress Of The International Astronautical Federation, 5-11 October 1991) [3] "Standardization agreement, characteristics of the global position determination system NAVSTAR (GPS) "(NATO, STANAG 4294) [4] GLONASS Approaches Full Operational Capability (FOC) "by P. Daly (ION GPS, September 1995) [5] "Space Vehicle Technologies and Technologies Module 6. Spatial Location "(Cépaduès Publishing) j6] "Low-Orbit Navigation Concepts" by H.James Rome (Vol 35, No. 3, Fall 1988, pages 371-390) j7] "GPS Techniques For Navigation Of Geostationary Satellites "by P. Ferrage, JL Issler, G. Campan and JC Durand (ION GPS, 12-15 September 1995) [8] "Applicability Of GPS-Based Orbit Determination Systems To A Wide Range of HEO Missions "by J.
- ~ Potti, P. Bernedo and A. Pasetti (ION GPS, 12-15 September 1995)

Claims (4)

18 1. Autonomous reduction method of thresholds acquisition and tracking of pseudo-random codes spread spectrum received in orbit by a receiver, edge of a satellite, accessing an orbital navigator, in which the receiver has a phase loop and a code loop, said method comprising the step of:
reception in the receiver from the browser, a rough help needed in an assisted acquisition mode, allowing the receiver to receive any signal with a signal to noise C / No such that C / No> (C / No) a, in which (C / No) a is a receive threshold in the assisted acquisition mode;
a .delta.PV uncertainty on a prediction of an aid from fine speed (pseudo-speed) respecting the condition next :
where B FI is a prediction band, fi a frequency of a carrier signal transmitted by a transmitter n ~ i, and C the speed of light, said process further comprising the steps of:
continuation of pseudo-random codes assisted by the help of fine speed provided by the orbital navigator and correcting an error between real and calculated; and lowering of thresholds for hooking pseudo codes to a value (C / No) avf in which (C / No) avf is a threshold for receiving pseudo-random codes in the acquisition mode assisted by the help of fine speed, a search for a phase of the code received around a phase prediction maintained by the speed aid fine from the orbital browser. 2. The process according to claim 1, characterized in that it includes a preliminary step during which the receiver starts cold without any help or external or internal message, and hangs any signal with a ratio C / No such that C / No> = (C / No) na, (C / No) na being a snap threshold in unaided mode. 3. The process according to claim 1, characterized in that in case of maneuver dedicated to an orbit control of the satellite, the orbital navigator receives a description the maneuver, and updates a speed aid provided by the browser, and in that the following condition is respected.
where .delta.PV is a precision on a pseudo-speed provided by the browser, in the absence of maneuver, where .delta.V is a precision on the description of the maneuver, where B FI is the prediction band, C is the speed of light and fi the frequency of the carrier signal transmitted by the transmitter no i. 4. The process according to claim 1, characterized in that when a pseudo-random code is pursued with a C / No report such as (C / No) with <C / No <(C / No) a, updates of the characteristic parameters of the transmitters are communicated to the receiver using remote controls External.