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In this paper, relative orbit of Low Earth Orbit satellites is determined using only GPS\udmeasurements and the effects of Equatorial Spread-F (ESF), that is one of biggest\udionospheric irregularities, are investigated. First, relative orbit determiation process\udis constructed based on doubly differenced GPS observations. In order to see orbit\uddetermination performance, relative orbit of two GRACE satellites is estimated for\udone month in 2004 when no ESF is observed. The root mean square of the achieved\udbaselines compared with that from K-Band Ranger sensor is about 2 ~ 3 mm and average of 95% of ambiguities are resolved. Based on this performance, the relative\udorbit is estimated for two weeks of two difference years, 2003 when there are lots\udof ESF occurred, and 2004 when only few ESF occurred. For 2003, the averaged\udbaseline error over two weeks is about 15 mm. That is about 4 times larger than the\udcase of 2004 (3.6 mm). Ionospheric status achieved from K-Band Ranging sensor also\udshows that more Equatorial Spread-F occurred at 2003 than 2004. Investigation on\udraw observations and screening process revealed that the ionospheric irregualarities\udcaused by Equatorial Spread-F gave significant effects on GPS signal like signal loss\udor enhancement ionospheric error, From this study, relative orbit determination using\udGPS observations should consider the effect of Equatorial Spread-F and adjust orbit\uddetermination strategy, especially at the time of solar maximum

The Effect of Equatorial Spread F on Relative Orbit Determination of GRACE Using Differenced GPS Observations