Why do dual/multi-frequency GNSS have stabler and faster fix than single frequency GNSS?


#1

Hi guys, I am aware that it is well known fact that dual- and multi-frequency receivers generally obtain RTK fix much faster than single frequency, and even float solutions are more accurate compared to single frequency GNSS.

Might anyone be able to share/explain the reason behind this?

My understanding is that the when resolving integer ambiguities for both frequencies, having multiple frequencies results in a narrower range of possible integer solutions for both frequencies, might this be correct?

Also, is this the same thing as ionosphere-free linear combination that I think is often mentioned for PPP?

P.S. I love reading up on the science behind GNSS but am not an expert by any measure, pardon me if some terms are used incorrectly


#2

Having 2 identical satellite transmissions on separate frequencies makes for a way to calculate the ionospheric delay. That removes a variable from the equation that solves the integer ambiguity. Basically:

2 or more frequencies = less variables in equation = solved faster = quicker fix

(also: = more complicated hardware = more money :wink: )


#3

Haha more money indeed! But that was very clear, thanks for the explanation!

I still wonder though, how much faster the fix is with specs that I often find listed on models of other brands - (GPS L1C/A,L2C, L2E, L5; GLONASS L1C/A, L1P, L2C/A, L2P, L3; GALILEO E1, E5a, E5b; BDS B1, B2; SBAS L1C/A, L5). Seems like a lot of overkill for very diminishing returns, if any. Especially since the RS2 already does so well in foliage and long baselines! :thinking:


#4

Let’s put it this way. With single frequency (M+, RS+) the time to acquire a fix is about as long again as the time it takes to boot up. With dual frequency (RS2), the time to acquire a fix is almost immeasurable compared to the time it takes to boot up.

I can’t compare to other brands (for lack of knowledge).


(Michael Lambert) #5

From what I have timed on average it takes an RS+ about 5 minutes to boot and collect the available constellations to a stable point that a fix can be obtained. That doesn’t mean the fix is going to happen though and this is exactly the factor you are talking about. Our Topcon Hiper V’s and VR’s take about 2 minutes to acquire the minimum requirement of 8 satellites and fix immediately.