# A Real-Time On-Orbit Signal Tracking Algorithm for GNSS Surface Observations
## 2 Background and Algorithm Overview
### GNSS-R Signal Tracking Overview  
Navigation GNSS receivers in personal navigation devices (such as in cell phones) are efficient at acquiring and tracking direct GNSS signals (i.e., from a direct and unobstructed path between the transmitter and receiver) for use in determining the position, velocity and time at the receiver. Various GNSS signal tracking techniques exist and are described in detail in several GNSS specific books [1; 21]. When tracking a direct signal, GNSS receivers are able to maintain a tight lock on the transmitted signal code phase and carrier phase for multiple parallel signals captured from a skyward oriented antenna. However, there are several complications to this standard receiver tracking technique that prevent or severely complicate the acquisition and tracking of an Earth reflected signal using existing tracking algorithms. These differences include,1. The delay and Doppler characteristics of a reflected signal vary significantly from surface to surface, causing significant variations in how the signal spreads in the delay and Doppler dimensions.
2. The surface reflected signals are often very weak and can fluctuate wildly in magnitude (especially over land) due to the surface scattering conditions, often requiring extended non-coherent integration intervals for detection.
3. Diffusely scattered signals often contain no information on the transmitted carrier phase, making traditional carrier phase tracking loops unreliable.  
These characteristics of surface reflected signals require spaceborne (and aircraft) GNSS-R instruments to have an on-board real-time tracking algorithm capable of estimating the delay and Doppler reference centers at the reflection point for the generation of delay Doppler maps (DDM) of received signal power, which are used to estimate surface characteristics, such as near surface ocean wind speed [17].  
By comparison, open loop tracking techniques for GNSS-RO receivers face a similar problem in that as the GNSS signals enter deep into the atmosphere, closed loop tracking techniques are not sufficient. In the case of RO receivers, it is often the case that on-board models are used to forward predict the signal path and open loop sample the received signal [2].  
There exist, in the public literature, a number of algorithms which partially describe GNSS-R open loop tracking techniques, including [20; 22; 23; 24]. These existing publications provide a starting point and a good overview of the basic concepts involved. However, the GNSS-R open loop tracking algorithm presented here expands on the existing state of the art in several important ways,  
1. This manuscript presents the first concise and self contained GNSS-R open loop tracking algorithm intended for general public use by the larger GNSS-R community world wide.
2. The real time performance of this algorithm has been validated with a large set of on-orbit data and analyzed with respect to its feasibility for real-time implementation (i.e., speed of convergence and spacecraft processing resources).
3. A land height correction technique is presented which will enable this algorithm to be run over low-lying land surfaces in addition to near ellipsoid ocean surfaces.