Under The Hood

	Under the hood is JPL's GipsyX/RTGx software. A precise point positioning (PPP) algorithm is implemented within GipsyX/RTGX to process GPS carrier-phase and pseudorange measurements in RINEX format. https://doi.org/10.1016/j.asr.2020.04.015 The GPS orbit and clock states are held fixed to the best solutions available from JPL. The following parameters are estimated: Position coordinates (a single set in Static Mode, or a time series in Kinematic Mode) Receiver clock states as white noise with updates every measurement epoch Zenith wet delay as a random walk with variance of 3 mm2 per hour Wet delay gradient as a random walk with variance of 0.3 mm2 per hour Carrier-phase ambiguities as real numbers
	The reference GPS orbit and clock states are of three types: JPL's Final products (also known as FlinnR), available about 10 days after data is collected with User Range Error (URE) of approximately 3 cm RMS. Clock solutions are provided every 5 minutes. These products are publicly available on a daily basis from https://sideshow.jpl.nasa.gov/pub/JPL_GPS_Products/Final. JPL's Rapid products (also known as QuickLookR), available about 1 day after data is collected with URE of roughly 5 cm RMS. Clock solutions are provided every 5 minutes. These products are publicly available ona daily basis from https://sideshow.jpl.nasa.gov/pub/JPL_GPS_Products/Rapid. JPL's Real Time products, available 1 minute after data is collected with URE of about 8 cm RMS. Clock solutions are provided every minute. These products are available as public service to the community.
	All of the reference orbit and clock products are defined in ITRF20. The positioning solutions, therefore, are also in ITRF20. By default the most accurate orbit and clock products are used if available. Since uploaded RINEX files are allowed to cross day boundaries, it is possible that several types of orbit/clock products are used in the PPP process. For example, several days of Final products followed by days of Rapid products. The JPL Final and Rapid orbit and clock products are produced on a daily basis, and span 30 hours centered on noon of the relevant day. When combining two consecutive daily products, cosine smoothing is carried out on the 6 hour overlap period to ensure continuity. In addition, the clock product of the second day is aligned with the reference clock of the first day. This process continues with any additional days. APPS users may specify the elevation angle cutoff, and for kinematic positioning the output data rate. By default, the 7.5 elevation angle cutoff is applied, and for kinematic positioning the positions are output at the same rate as the measurement data. The satellite clock solutions are interpolated if necessary. APPS automatically applies satellite and receiver antenna phase center variation (APV) maps following IGS standards. The choice of receiver APV maps is determined by the antenna type as it appears in the uploaded RINEX file. If the antenna type is not specified or is not recognized, APPS will not apply receiver APV maps. For single frequency data APPS applies ionospheric calibrations based on JPL's Global Ionospheric Map (GIM) data, and on Real Time Ionosphere (RTI) data, depending on availability. The GIM ionospheric data is typically available with a latency of a day, and the RTI data is typically available with a latency of 5 minutes. Certain data pre-processing and adjustments are carried out before the PPP is attempted, based on the receiver type as identified in the RINEX file header. These may include: Time tags adjustments C/A - P bias calibration
	If the receiver type is not recognized, only time tags adjustement is carried out by default. The following signal and ground models are applied by GIPSY: GPS yaw attitude [Bar-Sever, 1997] Phase windup [Wu et el., 1993] GMF troposphere mapping function Apriori hydrostatic delay = 1.013 * 2.27 * exp(-.000116 * h) meters, where h is the station height above the WGS84 ellipsoid in meters Apriori for wet delay is 0.1 m Tropospheric gradients [Bar-Sever et al., 1997] Relativity Pole, and solid tide [IERS Tech Note #23] Ocean tides [Desai] Second order ionospheric delay [Kedar et al. 2003]
	APPS produces many output files, which can be classified into two types: GIPSY output, and generic output. The GIPSY output contains file types and products familiar to GIPSY users, and includes: STACOV file, containing the position solutions and attendant covariance TDP file, containing time series for all estimated parameters with their formal errors
	The generic output includes: Tabulated time series for all estimated parameters A summary of PPP results A README file containing detailed description of the format and content of all output files
	APPS processing is carried out on dedicated computers at the JPL Global Differential GPS (GDGPS) System's Operations Centers. GDGPS has Operations Centers in Los Angeles, Pasadena, and Denver. These centers are operated 24/7.
	References: Bertiger et al., 2020, GipsyX/RTGx, A New Tool Set for Space Geodetic Operations and Research, for a detailed set of use cases, expected accuracies, and overview of the software. Bar-Sever, Y.E., "A New Model for GPS Yaw Attitude", Journal of Geodesy, 70, pp 714-723, 1996. Bar-Sever, Y.E., P.M. Kroger, and J.A. Borjesson, "Estimating Horizontal Gradients of Tropospheric Path Delay with a single GPS Receiver", J. Geophys. Res., 103, pp 5019-5035, 1998. Wu, J.T., S.C. Wu, G.A. Hajj, W.I. Bertiger, and S.M. Lichten "Effects of Antenna Orientation on GPS Carrier Phase" Manuscripta Geodaetica, Vol. 18, No. 2, pp. 91-98, 1993. Kedar S., G.A. Hajj, B.D. Wilson, and M.B. Heflin, "The effect of the second order GPS ionospheric correction on receiver positions", Geophysical Research Letters, Vol. 30, NO. 16, 1829, 2003. Zumberge, J. F., M. B. Heflin, D. C. Jefferson, M. M. Watkins, F.H. Webb, "Precise point positioning for the efficient and robust analysis of GPS data from large networks", J. Geophys. Res., v 102, No. B3, pp 5005-5017, 1997.
	FAQ What is the difference between APPS and AG? AG can only perform Static positioning with dual frequency measurements AG can only accept email requests, one file at a time AG uses JPL's legacy Final product (FlinnP), which is less accurate than the FlinnR product used by APPS AG can only process a single day at a time
	Can I still continue to use AG? APPS has fully replaced AG since September 2008.
	What types of input measurement files does APPS recognize? APPS recognizes RINEX measurement files of Version 2.11 and Version 3. The files may be compressed (Hatanaka or standard) or uncompressed. The file name is ignored. The files may span more than one day. APPS will attempt to extract receiver type for the RINEX file header.
	What receiver types are recognized by APPS? APPS recognizes the receiver types listed in https://sideshow.jpl.nasa.gov/pub/gipsy_products/gipsy_params/GPS_Receiver_Types.gz