# An Instrument Error Correlation Model for Global Navigation Satellite System Reflectometry
## 2 Materials and Methods
### Verification Techniques
#### 2.3.1. Curating Matchup Observations  
As a constellation of eight small satellites in the same orbital plane, CYGNSS is generally unable to collect collocated, near-simultaneous observations from different satellites. If the CYGNSS observatories were equally distributed across the orbital plane, each asset would follow the next at approximately a 10 min lag. However, in that time, both of the uniquely defining features of the CYGNSS observation changes: (1) the surface of the Earth rotates underneath the observatories, changing the ground track, and (2) the GPS satellites that serve as the source of radar signal advance in their orbit. Therefore, in 10 min, it can be quite challenging to develop one-to-one matchup conditions suitable for investigating the correlated error structure at timescales of seconds to minutes.  
CYGNSS has no onboard thrusters, and orbit phasing is controlled solely through differential drag. At several junctures during the CYGNSS mission, one of the observatories advanced within the plane to be nearly overlapping with another yet at a slightly different altitude. Generally, the greater the altitude difference between the observatories, the faster the relative precession within the orbit plane.  
An exhaustive review of each observatory's ephemerides for the life of the mission identified a few matchup opportunities. Matchups near the beginning of the mission were preferred, as the orbit planes of the satellites tend to drift apart over the lifetime of the mission. After further filtering by the operational status of each observatory to ensure that they were in similar attitude configurations and operating in similar science modes, a 24 h period starting 0Z on 11 SEP 2019 was chosen, where FM1 and FM5 were in a nearly identical orbital phase for a sustained period. A representation of the ground sampling during this period is shown in Figure 1.  
Each 'track' of observations (when a series of observations are made in close succession sharing a CYGNSS receiver and GPS transmitter) was matched sample-for-sample between FM1 and FM5 by first minimizing the distance between observations and then quality controlling for several factors:  
* Matched tracks must both contain more than 300 samples;
* Individual sample matchups are valid if samples are within 0.5 degrees (great circle distance);  
Figure 1: Comparison of FM1 and FM5 observations for 11 SEP 2019. (**a**) Rendering of the ground samples captured during the 24 h period in the near-overlap condition; (**b**) A single track of samples for both FM1 and FM5 that has been matched sample-for-sample to facilitate one-to-one comparisons of the observations. The samples from each observatory are no more than 0.5 deg apart in distance and were acquired approximately 3 s apart in time.  
Individual samples are screened to ensure no quality control flags apply; and
* The matched track is only valid if 60% of the data remains after all other matchup criteria apply. The resulting matchup conditions produced 103 matched tracks within the 24 h period of near-simultaneous observations.