How to do accurate geolocation using only public orbits (NORAD)
It is well known that accurate satellite carriers geolocation needs accurate orbits. Usually, geolocation system users ask to the satellites operator to provide these good quality orbits.
In some cases, these orbits are not available. What can the user do in this case ?
NovaGrid has developed a method called "co-orbit estimation" which estimates a coupled orbit of the couple of satellites using known location carriers (secondary references). This co-orbit allows to make accurate geolocations. This accuracy is quite the same than the accuracy we could obtain using operator orbits.
The following video shows two HyperTask missions:
- The first one is the co-orbit mission. This mission estimates the satellites co-orbits using one main reference carrier and two secondary reference carriers. This co-orbit is available through RPC requests.
- The second mission is a massive and automated geolocation mission. The orbits used are those provided by the first mission.
Simulatenous Multi-Target Geolocation Without Reference
Actually, the geolocation process using TDOA and FDOA measures needs to use a known location carrier (which is usually called the "Reference carrier") in order to compensate the satellites on board OL drift and accuracy.
In some situations, the user of a geolocation system works with a couple of satellite where no reference carrier is known. What are the solutions provided by modern geolocation systems in this case ? Usually, the suggested solution is to make two measures (several hours is needed between each measure) and to cross the two TDOA line of points (one line for each measure). While it is a good idea when one of the two satellites has an inclined orbit, when the two satellites have stabilized orbits this method gives a very poor geolocation accuracy (several 100 km) for the following reasons:
- the TDOA lines offers very small angular variations during the day (typically 0.1 degres )
- the limited accuracy of both TDOA measure and satellites orbits will move the estimated TDOA line (typically in the longitude direction)
In order to answer this kind situation, NovaGrid has developped an original method called "Simultaneous Multi-target Geolocation Without Reference" which is relevant when accurate orbits are available for both main and mirror satellites. The procedure is the folowing:
- Selection of the satellites (and orbits) used and the transponder of interest
- Selection of 5 to 15 targets to locate
The following video shows HyperTask in real life running a "Simultaneous Multi-target Geolocation Without Reference" task.
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New geolocation method without reference carrier
Standard satellite carrier (or interferrer) geolocation using TDOA / FDOA method always needs a reference carrier in order to compensate FDOA bias of satellites on board local oscillators.
When several reference carriers are known (called secondary references), the geolocation accuracy can be improved up to several kilometers using HyperSuite specific methods like multi-reference or co-orbit estimation. These methods also gives very accurate locations using public satellite orbits like NORAD TLEs.
NovaGrid has developped a new geolocation method using TDOA / FDOA technic which do not need any reference carrier. This method needs:
- Very accurate satellite orbits
- A sampling duration between 10 and 30 minutes.
Due to this large sampling duration, this new method cannot be used for interferer geolocation which are detected during several seconds. However, it can be used for locate some carriers which will become reference carriers.