OK, let me ask my question a different way:
Let's say you wanted to generate GPS type data in lunar orbit. Could you embed GPS data in future lunar satellite transmission systems so that a rover on the ground would not have to carry both a GPS receiver and a standard comm link, one transmission path could handle both datasets?
I think I saw the quote somewhere on NSF once where they mentioned that a GPS satellite was about as small as it was possible to shrink the package. If they could shrink it smaller, they would have.
To do "GPS" around the moon or another planet you need an accurate time source.
Around the moon you run into another problem, the moon's lumpy gravitational field. To be able to accurately use GPS you need to know precisely where the time signal originated from. That requires precise information on the satellites orbit. The moon's lumpy gravity makes that more difficult.
There are a few other options, and not all require a precise clock.
1. Instead of being passive, have an active system. Meaning you send a signal to the satellites and listen for a response from the satellites. You can determine precise distances to each of the satellites whose location is known. You do need to take into account the latency of the satellite receiving your signal and sending a message back to you, but that should be a constant. This then results in you only needing three satellites for "real time" positioning. But you need a transmitter and receiver.
*If you don't need real time, since a satellite will change position with time, multiple pings and knowledge of a precise orbit will allow you to determine your location to a high degree of accuracy.
2. If you are talking rovers on the moon, or Mars, or Comet ISON for that matter, ground based navigation beacons could work. You really only need two or three. It would provide coverage over a limited area, but it's not like anyone will be doing a Cannon Ball run across Mars in the near future. A "human" rover would not be roving very far from it's home base. You don't really need global coverage. I can even think of how to embed a precise time signal into the system without an atomic clock...
3. You could look up Transit. It was the first satellite navigation system. It worked on the principle of precisely knowing the satellites orbit and measuring the Doppler shift of it's radio beacons. It was not realtime, allowing the end user to get a location fix only after the satellite completed a ground pass. I would assume twice a day. But it was good enough for Polaris.
The real issue is GPS like capabilities are costly and several options exist for localized navigation. The GPS advantage is it is 24/7, mindless, and accurate. That capability does not come cheap.
Around the moon you run into another problem, the moon's lumpy gravitational field. To be able to accurately use GPS you need to know precisely where the time signal originated from. That requires precise information on the satellites orbit. The moon's lumpy gravity makes that more difficult.
Are very high lunar orbits that impacted by the lumpiness? For a spacecraft in a 10,000 km orbit, are perturbations that great?
BTW, is there "Extra credit" for no atmosphere on the Moon?
3. You could look up Transit. It was the first satellite navigation system. It worked on the principle of precisely knowing the satellites orbit and measuring the Doppler shift of it's radio beacons. It was not realtime, allowing the end user to get a location fix only after the satellite completed a ground pass. I would assume twice a day. But it was good enough for Polaris.
Since exploration systems really don't need to know where they are 24/7, I presume that a Transit-type system might be useful for lunar exploration. Perhaps a single satellite that used laser communications with the Earth so that its position were known, and then which transmitted that positional data with timing to the user on the lunar surface could provide daily positional updates to rovers.
Assuming that Elektra communications did not preclude this, a "Mars navigation" satellite could do the same at Mars and transmit positional data to Mars rovers using the Elektra radio system. The receiving spacecraft could extract positional data from the Elektra data stream, assuming that Elektra does not use some sort of fixed digital protocol. In that way, rovers at Mars would not have to include two receivers, one for commands and the other for navigation.
I guess I should mention that the requirement for positional information could be for any surface where there is a rover or lander or prospector, etc.
Are very high lunar orbits that impacted by the lumpiness? For a spacecraft in a 10,000 km orbit, are perturbations that great?
The mascon perturbations are much smaller in higher altitudes, but then you have more perturbations from the Earth. Nonetheless, I think by this point both of those are sufficiently well characterized that it wouldn't be a significant problem for navigation.
In response to your previous question: yes, IMO it makes a great deal of sense to combine communication relay capability with navigation for constellations orbiting around the moon and other planets.
Are very high lunar orbits that impacted by the lumpiness? For a spacecraft in a 10,000 km orbit, are perturbations that great?
The mascon perturbations are much smaller in higher altitudes, but then you have more perturbations from the Earth. Nonetheless, I think by this point both of those are sufficiently well characterized that it wouldn't be a significant problem for navigation.
In response to your previous question: yes, IMO it makes a great deal of sense to combine communication relay capability with navigation for constellations orbiting around the moon and other planets.
Thank you.
Now I have to find a Mars Q&A to figure out if navigational data can be incorporated into the Elektra data stream.
Now I have to find a Mars Q&A to figure out if navigational data can be incorporated into the Elektra data stream.
Not autonomously. Navigation data is produced on earth and uplinked. That is the way TDRSS works.
Now I have to find a Mars Q&A to figure out if navigational data can be incorporated into the Elektra data stream.
Not autonomously. Navigation data is produced on earth and uplinked. That is the way TDRSS works.
TDRSS receives navigation data for its own use, or does TDRSS re-broadcast it?
Please remember that the GPS constellation requires constant orbit determination by the controllers, to feed back that info into each bird. I understand that the onboard atomic clock are constantly checked for consiatency and accuracy. Thus, you need a lot of ground support to keep a GNSS system as accurate as current implementation. And the atmospheric diaturbance can be eliminated by using two, different frequencies.
So, you'd lose a lot of your accuracy. And for slow moving targets, like a rover, it's probably easier to just use the startracker. You know, like sailors have been doing for centuries.
I believe the word is sextant
TDRSS receives navigation data for its own use, or does TDRSS re-broadcast it?
TDRSs are just orbital tracking stations. They provide ranging data to the ground which processes it and uplinks a state vector to the spacecraft that needs a nav update.
