ADR Sign Convention used in NovAtel GPS Receivers and RINEX Almanac Injection Procedure Amperage vs Voltage ASCII to binary log conversion example Can carbon fiber be used as an antenna cover? Can the GPGGA positions be output with more decimal places? Destination IP Should be Set in ICOMCONFIG if Setting up UDP Connection Difference Between Message Latency and Time Accuracy DL-4 DL-V script Example Do GPS-700 series antennas have a defined "North point"? GNSS-750 MTBF GNSS Ephemerides and Almanacs GNSS Frequencies and Signals GPS Receiver and SPAN SDLC card synchronization GPS Tracking Performance on Aircraft (Power Supply Issues) How does RTKANTENNA work? How Electrical Overstress damage is caused How often does Averaging Filter in POSAVE Command get Applied? How to extract 85-bit string from GLORAWFRAME log How to Recover Abbreviated ASCII Logs from Raw Data How to Use PASSCOM to Redirect Data In what frame is CORRIMUDATAS output in OEM6? ITK and Dual Antenna Receivers Logs required by NovAtel Convert4 to generate a complete RInEx file Maximum Cable Length vs. Data Rate MIC Mating Connectors MSM 6 and MSM 7 Issue MTBF Specifications for GPS-700 Series New ALIGN Message Lengths OEM4 and OEMV Reporting 1994 (Week Roll Over) OEM615 MTBF OEM628 MTBF OEM6 Dev Kit weight OEMStar API Specifcations OEMStar MTBF OEMV1-DF MTBF OEMV-2 & OEMV-2G MTBF OEMV Auth Codes for 3.907 ProPak6 FTP ProPak6 - USB Drive File Format Recommended Alignment Procedures Recommended Logs for Post Processing Regarding Absolute and Relative Antenna Calibrations Regarding standard deviations used in RBV Calibration Remaining memory on OEM638/ProPak6 Reporting a Stolen Receiver RTK Quality Mode, Extra Safe Mode SBAS Position Mode Sending Batch File and Logging Binary Data in Real Term SPAN-CPT Commonly Asked Questions SPAN-CPT Power specifications SPAN Troubleshooting logs Updating SMART6 firmware from 6.230 or 6.231 to 6.510 (or later) USB Not Supported in WinLoad Software Using RTCM ephemeris data to speed up Time To First Fix (TTFF) WGS84 to ETRS89 datum transformations What Ionospheric Correction will SETIONOTYPE AUTO use? What is a EURO card? What is the Difference Between Active and Passive Serial Connection in CDU/Connect? What is the Effect of Bad Lever Arms in the GNSS/INS Filter? What is the Maximum Accepted Latency in ALIGN? What would cause speed (Field 8) in GPRMC to remain constant for some amount of time? Why am I seeing Two Kinds of HEADING2 Logs when Using two PP6Ds in Relative INS? Why Can't I disable RXSTATUSEVENTA permanently? Why is there no decoding information on the CAN PGNs? Why is the standard deviation of the BESTPOS log larger than the spec on the data sheet? Support Search

GNSS Ephemerides and Almanacs

GNSS almanacs and ephemerides both form the navigation message transmitted by each satellite. The navigation message consists of 5 subframes, where each frame is made up of 10 words and takes up 6 seconds to download. The navigation message frame structure is as follows:

subframes 1 -3: ephemerides
subframes 4-5: almanac --> full almanac requires 25 pages to completely download. 

Thus one page of frames 1-5 takes 30 seconds to download. Since the almanac contains 25 pages, the total time required to download almanac is: 25 pages * 30 seconds/page = 750 seconds = 12.5 minutes. On the other hand, the ephemerides takes 6 seconds/subframe * 5 subframes = 30 seconds to download. Note that the almanac is the same for all satellites whereas the ephemeris is unique to each satellite.

What is the purpose of each one?

Ephemerides: 

  • contains information on week number, satellite accuracy and health, age of data, satellite clock correction coefficients, orbital parameters
  • valid two hours before and two hours after time of ephemeris (toe). The toe can be thought of as when the data was computed from the GNSS control segment
  • Used for real time satellite coordinate computation which is required in position computation

Almanac:

  • contains less accurate orbital information than ephemerides
  • valid for a period of up to 90 days
  • Used to speed up time to first fix by 15 seconds (compared to not having almanac stored)

Thus, the receiver is capable of computing a position without having an almanac present. The almanac helps out with fixing the satellites for the first time but that is about it. The ephemerides however are vital for positioning computation.

The ephemerides are also used for post-processing as like was stated earlier, the ephemerides allow for the satellite position computation which is required in order to use trilateration to compute the receiver's position. Thus, it is very common for clients to log the ephemerides of each constellation they are tracking with the receiver. For brevity we will just refer to the GPS ephemeris from the RAWEPHEM message.We recommend clients to log this message with the ONCHANGED trigger. Here is why:

Scenario #1: LOG RAWEPHEMB ONNEW

i.   GPS ephemeris for satellites currently in view (as well as those not in view but saved in NVM) will be output as soon as log command is sent.
ii.  New RAWEPHEMB message will be output every 30 seconds as that is how long it takes for a frame to be received
iii. The consequence of this is logging more RAWEPHEMB messages than necessary thus using up baud rate which could be used for logging other messages

Scenario #2: LOG RAWEPHEMB ONCHANGED

i.   GPS ephemeris for satellites currently in view (as well as those not in view but saved in NVM) will be output as soon as log command is sent
ii.  New RAWPEHEMB message will show up every time a new satellite is tracked. This message only contains information on this new satellite
iii. No new RAWPHEMB message will show up if a satellite lock is dropped (i.e: moves below horizon).
iv. New RAWEPHEMB message will show up for each satellite once the broadcast ephemeris contains a different TOE